HOW TO APPLY LIMEWASH

Traditionally LIMEWASH was the principal finish applied externally and internally to historic buildings, quite often applied directly to the masonry or brickwork and more commonly to pre-applied lime coatings (i.e. harling, plaster, render etc;). Although often thought of as a decorative coating, the limewash was first of all a protective layer to the lime coatings and masonry substrate. On new lime renders and plasters it unifies and protects the surface particularly while strength is developing within the new plaster.

As with all lime coatings, limewash is a breathable coating allowing evaporation of moisture and water vapour. Limewash is also a repairing material, being used to fill small shrinkage cracks on the lime coverings. Limewash can also be used in conjunction with various aggregates to make shelter coats for friable masonry and will act as a sacrificial protective coat.

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In its simplest form, limewash is lime putty diluted by water to turn it into a milky consistency.

Additives for Limewash

When used externally limewash requires regular renewal, usually every two years.  The addition of a binder such as casein, tallow or linseed oil will improve its durability.

Casein is an animal deriative and can be added to a limewash to act as a water repellent and to prevent dusting.

Tallow is the refined fat of a cow, pig or sheep and serves a similar function but is more commonly found.

Both casein and tallow should be incorporated in the lime slaking process.

Linseed oil is a traditional vegetable-based additive which is used as a binder and weatherproofer.  Its water shedding properties are considerable, generally a one per cent solution is all that is required i.e. 50ml to 5 litres of wash.  It is worth remembering that although a greater quantity of additive will increase the protection against the weather, it will also inhibit moisture movement within the wall.

Using Limewash Internally

Limewash is ideal for internal use as it is unaffectd by rising damp which continually breaks down most other paints.  However, it is best not to use additives such as casein, tallow or linseed oil as this will reduce absorption and hinder the evaporation of moisture within the wall.

Plain Limewash

Plain un-coloured limewash will take on the colour of the lime used; this can range from pure white through to gray or buff coloured.

Coloured Limewash

Generally earth PIGMENTS were used to colour the limewashes, most commonly ochre’s, but also siennas and umbers, which produced a range of yellows, reds and oranges. Broadly speaking these produced pastel shades, although deeper colours are not uncommon. Coal dust, ash, blood and ground stone dust have all been found as additives in historic limewashes to achieve the desired colour

Application of Limewash – Preparation
Surfaces to be limewashed must be clean, free from grease and they must be porous. Previously limewashed surfaces must be well brushed down and any loose limewash scrapped off. Any mould should be treated with fungicide and thoroughly washed off with clean water. Do not use fungicides, which contain silicon.

Damping Down

Limewash should never be applied to a dry surface, as this will cause rapid drying out of the limewash and result in dusting. SPRAY about 3 sq/m of the surface to be limewashed with water until the surface is damp but not running with water. Do not try to damp down the whole wall or ceiling at one time, as most of the area will be dry before it can be limewashed. Dry joints must be avoided as these will result in the limewash gaining a patchwork appearance.

Application

Limewash is best applied by using a flat brush or masonry paintbrush. Stir the limewash well before and during application, apply working the wash well into the surface. The limewash should be applied in several thin coats. Avoid runs or drips running down the face of the work.

The limewash will appear transparent when first applied so care must be taken not to build up the limewash too quickly as this will craze on drying. Each coat should be allowed to dry before the next coat is applied. We recommend 24 hours between coats. It is very important to rewet the previous coat before applying the next coat. At least 4 coats will be needed to cover new work. Each coat will need to be burnished into the surface with a dry brush as it starts to ‘gel’. This will give a surface free from brush strokes and leave a unified finish.

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As earth pigments are a natural product slight variations in colour do occur. We highly recommend when ordering coloured limewash order the whole amount required plus 15%, this should then be mixed together in a large container and will avoid variation in colour over the job.

Common Mistakes and Solutions

Limewash dry but powdery: Dried too fast, spray with water and re coat with limewash. Limewash not absorbed: Unsuitable non-porous surface remove and use alternative product. Limewash patchy Insufficiently mixed, mix following coat thoroughly. Limewash dries too quickly: Remove flaky limewash and damp down the background

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A practical guide to rendering and plastering with lime

Lime renders were traditionally applied to give protection to walls built of poor quality rubble stone or porous brick or to walls in exposed locations facing driving winds. They help by acting like a sponge, absorbing rainfall then allowing it to evaporate rather than soak into the wall. Most cottages and houses built of rubble stone would have been rendered originally and they tend to suffer from penetrating damp if the lime render is removed or replaced with a cement rich render.

There is a very wide range of types of lime rendering. Rubble walls of many vernacular buildings were often treated with just a single coat of render, amounting to not much more than a very full, flush pointing. Such a render is thicker in the hollows and very thin over the stone faces. There was no attempt to create a flat surface so the undulations of the wall and even some of the stones themselves were not concealed. For a smarter finish or on more prestigious buildings the aim would be for a more uniform render achieved by applying a scratch coat to fill the hollows and take up some of the unevenness followed by one or two more coats which were worked to a flatter surface. Sometimes joint lines were ruled into the damp top coat to create the illusion of ashlar stone, but a common finish for many houses and cottages was a rough-cast where the final coat consisted of a mortar slurry containing coarse grit applied by throwing from a special trowel. For interiors a fairly smooth surface could be obtained using a coarse render mix, but for top quality internal plastering the final coat would be richer in lime and polished up to a smooth, close finish.

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Renders and plasters can be applied to a variety of backgrounds including earth (which should nearly always be rendered), stone and brick. Plaster is also applied to wooden laths for ceilings and internal partitions.

By carefully selecting appropriate aggregates it is possible to match existing renders and successfully repair failed patches without the need for complete re-rendering. Hollow or detached plaster can sometimes be consolidated and saved and further advice should be sought before replacing it, especially if it is very old.

Preparing the wall
For stone and brick any hollow or decayed render should be hacked off and any loose pointing should be raked out and replaced prior to rendering. Brush the wall to remove loose material. Do not rake out pointing to provide a key. Do not use plastering bead on corners as this will give a modern appearance. Do not use chicken wire or metal lath to form a key as it can cause stress in the render due to differential thermal movements and can lead to large-scale failure, especially when it rusts.

For plastering onto existing wooden laths check that they are firmly fixed and free of lumps of old plaster. New laths should ideally be riven oak or chestnut. Sawn laths are inferior as they are smoother and weaker than those split along the natural grain of the wood. Laths should be fixed so that the distance between them is approximately 8-10mm. This allows the right amount of space for the plaster to be pushed between the laths and flop over to form a key. Do not apply preservative treatments to either old or new laths as they can introduce harmful salts into the plasterwork. Metal lath is sometimes used internally instead of timber laths as it is quicker to fix and cheaper, but it is harder to plaster onto as it is slippery and the sharp edges may cut into and weaken the plaster key. Plenty of hair in the mix is essential.

For masonry, thoroughly wet the wall with clean water using a hose-pipe or . The more porous the background the more water will be required. Allow the water to soak in a bit then spray again, and repeat until the surface layers of the wall are thoroughly damp. When the renderFor stone and brick any hollow or decayed render should be hacked off and any loose pointing should be raked out and replaced prior to rendering. Brush the wall to remove loose material. Do not rake out pointing to provide a key. Do not use plastering bead on corners as this will give a modern appearance. Do not use chicken wire or metal lath to form a key as it can cause stress in the render due to differential thermal movements and can lead to large-scale failure, especially when it rusts.

Mixing plaster and render
A basic lime render or plaster can be made using the same ingredients and mixed in the same way as a pointing mortar. A coarse sand can still give a smooth finish suitable for most vernacular buildings although the mix should be slightly richer in lime than a standard pointing mix, say one part of lime to two-and-a-half or three parts of sand. Do not be tempted to use a soft, fine sand – you will just end up with lot of cracks. The mix will need to be slightly wetter than for pointing but it should be slightly stiffer than its modern cement or gypsum counterpart. Once you start plastering you will soon discover whether your mix is the right consistency: too thick and it will be virtually impossible to apply it smoothly and get it to stick to the wall; too thin and it will go on beautifully then slump, sag or drop off.

Applying-Lime-Plaster

Whether you mix your own or buy ready mixed lime render or plaster it is a good idea to ensure that the lime putty used is at least three months old. This will ensure that the lime is thoroughly slaked. If the lime is younger than this any unslaked particles in the mix may slake some time after plastering causing a small eruption or ‘lime blow’. Many practitioners advocate the use of six month old putty for plastering, but few suppliers stock it.

When plastering onto lath it is essential to add hair to the mix to help in forming a key of plaster between the laths. Whilst it is not essential to use a haired mix for other backgrounds it can help to reduce shrinkage. Hair should be added to the mix just before use. Do not add hair to coarse stuff that is to be stored for more than about six weeks as the hair may rot if left in damp lime mortar for a long time. The hair should be gradually teased into the mix so that it is well distributed and does not form clumps (often referred to as ‘dead mice’). As a rule add 4 Kg of hair per cubic metre of mortar for work on laths. For the second coat halve the quantity of hair. To check whether there is sufficient hair in the mix, scoop a dollop of mix into a gauging trowel, tap the underside of the trowel smartly against a hard edge so that the blob flattens and the surplus mortar falls off the edge of the trowel. There should be a fringe of hair at 1-2mm intervals around the edge of the trowel.

Applying Render
There are many different ways to apply render depending on the type of finish required, the type of lime used and the preference and experience of the person applying it. The following should give you an idea of some of the common procedures but is by no means the final word.

Rendering or plastering is not something that can be easily taught without a practical demonstration, but if you have already done some plastering or can get someone to show you how, there are several guidelines that will help you to use lime render successfully.

Lime renders shrink as the water in them evaporates. This can be minimised by using a well graded aggregate, by ensuring that the wall is well wetted before you start and by applying the render in thin coats of no more than half an inch. It also helps if the mix is as dry as possible but obviously it has to be wet enough to be workable and if you observe the other points you can get away with a slightly wetter mix which is easier to use.

If there are deep hollows in the surface of the wall dub them out first using lime mortar and small bits of stone or tile, and allow this to firm up before applying the first coat of render. There is no need to try to create an absolutely smooth flat surface as on most old buildings lime render and plaster looks best if it follows the contours of the wall.

Lime renders must be applied using as much pressure as possible to force the mortar into the surface crevices or between the laths to form a close contact between mortar and backing. For masonry walls, whilst it is possible to apply render using either a gauging trowel or a plasterers trowel the best result is achieved by throwing the mortar on from a trowel. This technique ensures the best bond between the mortar and the wall, expels any air in the mix and ensures that the mortar is well compacted. If you use a float or gauging trowel it is very difficult to apply the render with equal pressure all over the wall: it will tend to be under more pressure over the high spots and under less pressure in hollows and therefore more likely to drop off. Throwing render sounds difficult but it is surprisingly easy particularly for the scratch coat or dubbing-out coat, and involves less physical effort than using a trowel. This is particularly important if you are not used to plastering on a regular basis. It doesn’t matter if the first coat goes on rather unevenly as you can remove any excess mortar by running the edge of a trowel over the surface to cut off the rough bits. Just remember to protect windows, rainwater goods and any other areas that you do not want covered with lime mortar, including yourself, particularly your eyes. If you prefer to trowel the mix on you may find it easier to apply the mortar using a gauging trowel rather than a plasterers trowel as it is better for getting into the hollows and maintaining an even pressure over the entire wall.

Once you have applied the scratch coat and got a fairly flat surface, subsequent coats can be applied successfully using a trowel, although it is still easier to throw it on.

Tyrolean rendering machines which splatter mortar onto the wall do not achieve the necessary level of compaction of the mortar as it hits the wall and are not suitable for the application of traditional lime renders.

For plastering onto wooden laths a plasterers trowel is suitable, but you must apply the plaster with enough pressure to force the mix between the laths so that it can flop over behind the laths and form the key.

Use a clean tarpaulin or sheet of polythene to protect the floor or ground along the foot of the wall. You will then be able to scoop up and re-use any mortar that doesn’t stick to the wall first time. If you simply cannot get the mortar to stick try re-wetting the wall or experiment with a slightly wetter mix.

As you apply the render do not try to smooth the surface by stroking with a steel trowel or float. Working the surface of the wet mortar with a steel tool will draw the lime to the surface creating a lime-rich layer over a weak, lime-depleted layer which can lead to premature failure of the render. Simply apply with one stroke, pushing hard or throw it on. If there are high spots or ridges hold the edge of the trowel or float against the wall and draw it across the surface. This will cut off the rough bits and leave a good open texture.

As the mortar starts to firm up it may develop cracks, although cracking will be minimised if the points in the first paragraph are observed, and hydraulic lime tends to suffer less from shrinkage and cracking. Cracks in the base coats can be left as they will not compromise the strength of the plaster and will be covered by subsequent coats. However, you must make sure that the cracks are due to shrinkage and not because the coat is peeling away from the backing; push the coat gently to check that it is firm against the backing.

Create a key for the second coat by scoring the surface in a pattern of diamonds using a lath scratcher. Do not use the edge of a trowel as this will create too fine a groove to provide a good key. Some practitioners advocate throwing on the scratch coat and leaving it rough to provide the key for the next coat, but this depends on being able to throw the mortar on reasonably evenly in the first place or it will be almost impossible to render over it if there are huge humps and hollows. It is vital to take time and trouble over creating a good key as the adhesion of subsequent coats depends on it. Do not be tempted to miss areas in awkward places or at junctions with architraving or ceilings. Do not scour or float a scratch coat on laths. as this may damage the nibs.

There are two schools of thought regarding when to apply the second coat. Some practitioners advocate applying the second and subsequent coats whilst the previous coat is still ‘green’ or ‘leather hard’, that is after it has firmed up sufficiently to resist indentation with a thumb but is still soft enough to scratch with a finger nail and is still damp.This varies according to drying conditions. For internal plaster it may be anything up to a week or so between coats. This method requires less dampening of the surface before application of the second coat and may achieve a better bond between coats. However, there is a risk that there might be further shrinkage in the base coat after the second coat has been applied, and it will take a lot longer for the undercoats to fully carbonate and achieve full strength.

The alternative approach is to allow the base coat to dry out slowly and start to carbonate. The work should be protected from drying out too quickly by covering with damp hessian for at least one week, and often for two to three weeks according to conditions. By this stage carbonation will have started (but not be very far advanced) and there should be no further shrinkage in the base coat. The base coat needs to be thoroughly dampened down before applying the next coat. This method is more dependent on a good mechanical key between the coats, and requires a longer period for completion of the work than the first method.

In the second coat cracks should be closed up by scouring the surface with a wood float using a circular movement and pushing hard to consolidate the coat. This will also enforce the bond between the coats and remove the high spots creating a flatter surface ready for the next coat. You may need to do this several times until the mortar is firm and no further cracks develop. The importance of this scouring and consolidation process for the success of lime rendering cannot be over emphasised. It must be done thoroughly no matter how wrist-breaking it may be.

Timing is also important. For non-hydraulic lime the mortar must be firm enough that the scouring will not just re-work the mortar, but not so firm that the cracks cannot be closed up. This can be anything from several hours to several days depending on conditions.

Always ensure that the previous coat is damp before applying the next one. If applying a fairly fine top coat, keying of the previous coat is best carried out using a comb scratcher or a devil float which creates a finer key than a lath scratcher. If the keying is too coarse it may well result in cracking of the top coat along the lines of the key because the render will be comparatively much thicker in these places.

The final appearance depends on the type of mortar used for the top coat and the tools used to finish it. Scouring with a wood float will result in a fairly open texture suitable for the majority of external renders on vernacular buildings. For internal plastering a combination of wood and sponge floats and a plasterers trowel can be used to create a smooth polished finish. A traditional type of external render finish for simple cottages and farmhouses is known as roughcast. A slurry of mortar containing some quite coarse particles of gravel is thrown onto the top coat of render from a dashing trowel. This creates a rough texture with a large surface area which helps in allowing the wall to breathe, but requires some expertise to avoid a porridgey mess.

The work must be protected to prevent rapid drying. Both hydraulic and non-hydraulic limes must be kept slightly damp but at the same time air must be allowed to circulate. If using polythene or tarpaulin it should be fixed so that there is a gap between it and the wall. Remember that water will drain down through the render under gravity so the upper parts of a wall will start to dry out faster than the lower parts and you may need to spray these areas more frequently. Frost can be a particular hazard to a young render as it can cause damage weeks after the render was applied, especially if preceded by heavy rainfall. It is a fallacy to believe that by using hydraulic lime or a pozzolanic additive you can “beat the frost”. It is only the initial setting which takes place by hydraulic reaction and the mortar still requires a long period of time to carbonate and strengthen. If hard frost is forecast within a couple of months of application then ideally the render should be protected by hanging polythene, bubble wrap or hessian over it, although on most buildings this may be impossible. A render which survives its first winter unscathed is more likely to wear well subsequently.

Repairing Render
If patches of render have fallen off cut back the edges to sound plaster. The best tool for this is a craft knife. You can cut back using a bolster chisel but you risk loosening adjacent plaster. Ensure that the edges of the patch are cut square to provide a key for the new mortar, and eliminate feather edging.

Where render is cracked or hollow it may be possible to consolidate and save it and further advice should be sought before hacking it off, especially if it is believed to be very old. However if it is beyond repair or not worth saving hack it off, cutting back round the edges to sound plaster.

Rake out any loose pointing in the wall behind and repoint roughly. Brush out loose dust and apply the render in thin coats following the guidance above and taking care to ensure that it is well pushed in beneath the undercut edges of existing plaster. Apply the final coat with a neat butt joint between the old and new and with a slight bulge in the middle of the patch so that the final consolidation will create a flat patch, flush with the surrounding render. Do not feather the edge of the patch over the surrounding plaster. Push back and consolidate the patch with a wood float. For small patches it is best to use miniature wooden floats which can be made by fixing a small handle to a short piece of batten and rounding off the edges with sandpaper.

Internal Lime Plastering

Tim Ratcliffe

The promotion of modern gypsum-based plasters has led to the almost complete demise of lime plastering, and of many of the traditional skills associated with the craft. This has been exacerbated by the plastering trade being divided into flat and decorative work, with new ‘fibrous plasterwork’ being made in workshops. Many youngsters entering the trade are now just taught the basic skills to enable them to stick up plasterboard and skim plaster onto it. We are told that it is all down to ‘supply and demand’; if this is the case, those of us involved in work on old buildings need to be more demanding.

There is a real need for skilled plasterers who can plaster with lime, and also turn their hands to repairing and reinstating dado and cornice mouldings in situ. The current training system works against anyone gaining this set of skills.

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An article like this cannot resolve this skills shortage, nor attempt to even describe the range of skills that a traditional plasterer should have. There are, however, some general principles which anyone involved with lime plastering should be aware of. Sadly there are too many cases of lime plasters failing because the people who have specified the work or the people carrying out the work don’t have adequate knowledge or experience.

THE RIGHT MATERIALS?

Most people using lime in old buildings have a vague understanding of the benefits of a ‘breathing’ mortar or plaster, but if they perceive that lime is too difficult to use they may decide not to bother with it. We need to make it clear that the revival of the use of lime is not some ‘airy fairy’ idea dreamt up by a bunch of idealists. On the contrary, it is driven by the realisation that buildings are suffering because they have been coated with inappropriate materials, and the people living in them may be less healthy as a result. There is compelling evidence that modern gypsum plasters encourage condensation and consequent mould growth if used on walls that are supposed to ‘breathe’. We are beginning to see a revival in the use of lime plasters and we need to encourage a revival of the skills required to use them.

Having bemoaned the lack of proper training for anyone wanting to learn traditional plastering skills, it has to be said that lime plastering is not rocket science. With a basic understanding and a willingness to learn, most plasterers can pick up the skills required to produce a reasonable job in a few days. This is not the same as the skill required to repair plaster in a fine quality country house, or the experience required to match a range of historic finishes, but these things come with time, and we can hope that as more plasterers learn to use lime they will be inspired to develop their skills and understanding further.

There are two characteristics that differentiate lime plasters from modern plastering materials. The first is that they set slowly by absorbing carbon dioxide from the air, in the presence of moisture. The second is that they will shrink as they dry.

Although hydraulic limes, which set more quickly than white/fat limes, are occasionally used for plastering in damp conditions, they are less flexible and breathable than the latter, and their use internally should generally be avoided. All the evidence on old buildings and in written documentation indicates that for centuries, if not millennia, plasterers have chosen to use white/fat limes for internal plastering.

There is some debate about whether we really need to use traditionally slaked lime putty, or if bags of dry hydrated white lime from the builders’ merchant are just as good. Although chemically they are the same (both are calcium hydroxide), a traditionally slaked lime, which has been matured for three months, will have broken down into much smaller particles and started to form crystal chains. This gives it better adhesive qualities, helping it to grip the wall more tenaciously. The difference seems to be in the maturing process; so if recently hydrated lime from a fresh bag is left to soak in water for three months, it should be as good as a traditionally slaked and matured lime. However, in practice most of us find it is easier to buy the matured stuff from a specialist supplier.

APPLICATION

Lime plaster in most buildings from the second half of the 17th century onwards was applied in three coats, which enabled a flat finish to be achieved. In agricultural buildings two coats of plaster are common, or even one single coat may be found where an undulating surface was acceptable. Similarly, pre-18th century buildings often have undulating plaster finishes, and this usually indicates that fewer than three coats were used.

Before embarking on any plastering project it is worth assessing the number of coats used originally and/or the quality of finish required. On the basis that three-coat work is the most common in historic buildings, it is best to understand how to apply this and then reduce to two or one coats where appropriate.

SCRATCH COAT

The first coat is known as the ‘scratch coat’, because the surface is scratched with lines to give a key for the next coat. The mix used is usually one part lime putty to two and a half parts of coarse, sharp, well-graded sand. If the grading of the sand includes more or less of a particular grain size the amount of lime may need to be varied slightly. An experienced plasterer will be able to tell instinctively whether another half part of lime or sand needs to be added. Another way to tell is to take a sample of the dried sand and measure the volume of water required to fill all the voids between the grains; the amount used is equal to the amount of lime required.

Hair can be added to the mix to give it tensile strength. Although this isn’t absolutely necessary when plastering onto stone or brick, its benefit in the long term is that if the building moves or any patches of plaster detach from the substrate, the hair will help bridge over any gaps. Old plaster can sound hollow in places when tapped, but is usually still sound if it contains hair. When plastering onto laths, the addition of hair becomes a necessity, because plaster does not adhere well to timber once it has dried; it relies on interlocking fingers formed as the wet plaster squeezes though the laths and slumps over, so the tensile strength imparted by the fibres is vital.

It is important, before applying new lime plaster, to ensure that it isn’t going to be sucked dry by the background it is applied to, as this will cause it to shrink and potentially fail. The suction can be reduced and controlled by wetting the substrate before applying the plaster. If the wall is very dry and porous it may need to be sprayed with a hose pipe a couple of times on the day before, and then once again on the day of application, but if it is less porous and the environment is relatively humid, spraying with a hand-held spray on the day of application may suffice. There needs to be enough moisture in the wall for it still to be damp to the touch after an hour, but no longer glistening with droplets of water.

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The scratch coat should be no thicker than 15mm (5/8 inch). Any deep recesses or holes should be ‘dubbed out’ beforehand, using a stiffer (drier) mix, and allowed to dry to avoid deep pockets in the scratch coat. If plastering onto laths it is important to apply the plaster diagonally to the line of the laths, joining up each time with the previous area laid, to achieve a consistent key between the laths.

While still wet, the surface should be scratched with a three pronged lath scratcher or a single pointed lath (which is slower but gives a better job). The scratching should be in straight lines, diagonally to the laths or the line of the wall, in both directions, to create a diamond or lattice pattern. The quality of the scratching affects the keying of the next coat, so it should be done carefully to achieve an even pattern, and, on laths, particular care should be taken not to cut through to the lathing.

The scratch coat should then be left to dry and shrink before attempting to apply the next coat. In most circumstances it will need two weeks to dry out, but can take up to four weeks in some cases. Shrinkage cracks are likely to appear as it dries, but this is not a problem. The important thing is to avoid it drying too rapidly, which can cause it to fail. Exposed areas of plaster (adjacent to open windows, for example) may need to be covered with hessian or polythene, and the use of dehumidifiers should be avoided.

Once the pad of a thumb can no longer indent the scratch coat it is ready to take the next coat. At this stage the surface should still be slightly damp to the touch and will just need brushing down to remove any loose grains and then lightly dampened with clean water, using a hand-held spray. If it has been left too long and has dried out completely, more water will be required.

FLOATING COAT

The second coat is known as the ‘floating’ or ‘straightening’ coat, and is used to bring the surface to a level plane. The mix is usually slightly less rich than that of the base coat, typically one part of lime putty to three parts of coarse, sharp, well-graded sand, and normally without any hair. Again, it should not exceed 15mm (5/8 inch) in thickness. A level surface is achieved using long ‘floating rules’ or ‘straight edges’, passed over the wet surface to remove undulations.

In the best quality work, wooden blocks (known as dots) are temporarily applied and plumbed and levelled; lines of plaster (known as screeds) then join between the dots, and are levelled using a floating rule; finally the spaces between are filled using trowels and levelled with a floating rule with its ends bearing on the screeds. This method was used in finer quality Georgian and Victorian buildings.

Once it has begun to stiffen up, the floating coat needs to be consolidated by ‘rubbing up’ the surface using a wooden float to counteract shrinkage. This is likely to be required once or twice on the day of application, and may be necessary on the following day as well. The timing depends on the speed of drying. Sprinkling the surface with water, using a brush, assists the circular rubbing action if it has dried out too much.

Once the surface has been compacted, a ‘devil float’ (wooden float with nails or screws driven through the corners to project about 2mm) is rubbed over the surface to form a key for the finish coat. It should then be left for about a week or so before it is ready for the final coat.

FINISHING COAT

The final coat is known as the ‘setting’ or ‘finishing’ coat. It is usually thinner than the other two coats and uses a finer sharp sand. The mix can vary depending on the hardness and the type of finish required; the richest mix being three parts of lime to one of fine sharp sand, and the leanest mix being one part of lime to three parts of sand. More sand will give a harder finish and is more suitable for open textured float finishes; more lime will give a softer surface but allows it to be polished smoother. For standard work, a mix of one to one is suitable.

The thickness of this coat can vary between 2mm (1/16 inch) and 5mm (3/16 inch). In order to achieve an even finish on the surface, it needs to dry out consistently, so is applied in two or three very thin layers. The additional time and labour required for this is well worthwhile because it controls the drying of the surface and allows the plasterer to achieve the finish required.

Before applying the setting coat the floating coat needs to be lightly dampened with water to control the suction. Each layer is skimmed on as thinly as possible, working in alternate directions each time, and is laid over the previous one as soon as it has had a chance to ‘steady-up’ (usually in about half an hour). When the work is firm enough it should then be scoured to compact and consolidate the surface.

A straight-grain float (that is to say, one with the grain running the length of the float) can be used for the scouring process, but if a very flat surface is required, a cross-grain float is better. The ‘cross-grain’ prevents the edges wearing down and ensures that any projections are shaved off as the float passes over the surface. Cross-grain floats are only used to rub or scour over a surface and any specification that refers to using them to apply lime plaster is clearly wrong.

Depending on what is required, the surface can then be worked over using either a trowel, to achieve a fine closed finish, or a combination of wooden and sponge floats, to create an open textured finish. Some water is likely to be required in this process, splashed on with a brush.

The rubbing and scouring process required to achieve a suitable finish can present a problem when patching up to old plaster. The first important point to note is that the exposed edge of the old plaster is likely to suck more moisture out of the new plaster than the wall itself, so more water will be required to control this suction at the edges. Once the area has been patched, particular care will be required when rubbing over the setting coat, to avoid forming an indent at the junction with the old plaster.

n some cases, where an absolutely blemish free surface is required, the setting coat mix can be gauged with Plaster of Paris (a form of gypsum) to minimise the need for scouring. However, the decision to add another material needs to be taken carefully, and the visual compatibility of the repair needs to be balanced with its technical compatibility. This brings us back to the need for traditionally skilled plasterers who understand the materials they are working with. Without these skills we are lost.

Lime Mortar for use in Today’s Restoration and New Build Projects

Why use Natural Lime Mortar

The use of natural lime mortars and renders has increased considerably in recent years compared to just a couple of decades ago. These simple, basic materials are fundamentally important to the long term survival of historic buildings, yet there are many who still regard their use with suspicion. Natural Lime Mortars are the only viable way to repair and restore much of the great architectural heritage of this country.

Invariably, our Historic Buildings, Monuments and Structures will have been constructed with the Natural Lime ‘Products of the Era’. Attempts to repair these national treasures with modern ‘Cement’ instead of a compatible Natural Lime Mortar, will frequently produce a worse result than no repair at all. These ‘Bodged Repairs’ then have to be reworked first to undo the damage inflicted by the alien chemical nature of modern cements before they can be repaired using the Natural Lime Mortar and Render that should have been used in the first place.

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If you are involved in a repair or restoration project at the early planning stage you would be well advised to contact ‘Cornish Lime’ for advice on the appropriate Mortars and Renders for the repairs being undertaken. You may also like to attend one of our regular Lime Training sessions, a one day practical and theory course covering such topics as pointing, rendering, bag rubbing and plastering.

We have an extensive portfolio of prestigious projects where our expertise, advice and Natural Lime Mortar has formed the basis for extensive repairs and restoration including The National Trust and English Heritage. You may wish to view our information on Lime Putty and Coloured Mortars.

The Use of Natural Lime Mortar

Selecting the right Lime Mortar for your particular application can appear very daunting. That is where ‘Cornish Lime’ become not only a supplier, but a source of knowledge gained over decades of successful restoration projects using the same Natural Lime Mortars that they themselves manufacture today.

In addition to the freely given advice and comprehensive literature packs available, Cornish Lime also run regular, informative training sessions on the use of Lime Mortars, Renders and Plasters including the versatility of their own Mature Natural Lime Putty.

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One example of our Training is the course entitled: “Introduction To And Working With Lime” which consists of the following content:

  • Selecting the right materials
  • Compatibility and interaction of Lime Mortars, Renders and Plasters with other materials
  • Surface preparation and suitable or necessary base coats etc
  • Mixing the Lime Mortar, Render or Plaster
  • Useable life span of Limes after mixing
  • Curing times and protection during the curing stages
  • Adequate Q & A session time for you to explore your specific thirst for knowledge
  • Comprehensive handouts to keep for your own reference
  • Building with Lime Practical ‘Hands On’ session using the appropriate materials
  • Pointing, Rendering and Plastering with Lime
  • Sands and aggregates, the role they perform and their suitability
  • Decoration

Please Contact Us to find about more about our lime training and for availability on the next course.

 

COB RENDERING

Introduction to Cob Rendering

Cob walls should always have a well maintained roof/capping and where possible the plinth should be exposed and repaired re-pointed as necessary using suitable lime mortar. Or as the old saying goes the wall should have a good hat and boots to keep the moisture out.

A cob wall in good condition in a sheltered situation does not need render coats for protection. A render finish even lime can never look as good as cob.

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Cob, chalk and earth walls are by nature of a soft friable composition.

Rendering and repairs should be carried out with lime putty mortar – MEDIUM STUFF or SINGLETON BIRCH NHL2 mortar 2.5:1.

External lime coatings should be applied in relatively thin coats, therefore any defects in the form of hollows must be corrected before the application of the render coats by either rebuilding with cob material (normally available in the immediate area) or with cob blocks available from us.

Defects should not be corrected through coats of “dubbing out” (Thick applications of render/mortar). Areas of varying thickness are prone to shrinkage, carbonation and curing problems. Careful background preparation plays a vital part in the weather resistance capability of the wall.

Materials to be used in background repairs, should, wherever possible, be matched to the existing fabric. In doing so, the repair will be compatible and produce a similar performance pattern. Where previous remedial work has taken place with unsuitable materials, (i.e. dense cement mortars or bricks), these should be removed if possible without causing more damage and repaired with matching lime mortars or chalk/chalk cob.

Cob Wall Preparation
The background must be free from dirt, grease and vegetation. These elements should be removed before repairs are underway. The removal of biological growths should be thoroughly carried out, as any remaining lichen, algae, etc; will grow back and attack the bonding between lime finishes and the background.

Cob and earth walls have a friable surface which needs to be prepared before render or plaster can be applied. The wall surface should be wet down the day before application. On the day of application it should be dampened (but not wet). A harling coat is then applied to the wall by casting vigorously from a HARLING TROWEL (or even a coal scuttle shovel) It can also be applied by mortar spray or Tyrolene gun. It should then be left for at least 4 days to set hard before the application of the first scratch coat of render.

The harling material is a combination of aggregates and lime, mixed into a slurry consistency and applied by casting vigorously at the wall.

First Coat
The first coat of lime render is applied by use of a laying on trowel or float, the coat is applied to a thickness of approximately 8-10mm thickness, coats much thicker than this will result in shrinkage cracks, in 2 coat work, this coat will need to be straightened by use of plastering straight edges, once flat the coating should be left to stiffen up, sometimes known as tightening, as this happens, the render should be compacted and compressed by scouring the render with a wood or polyurethane float, i.e., rubbing up in a circular motion. The over working of the surface should be avoided as this will draw lime to the surface. During the scouring process any shrinkage cracks should be closed, by pushing the crack back with the edge of the trowel and rubbing new material into the depression.

Once this process has been completed the surface is keyed by use of a scratcher in a diagonal pattern. The inclusion of hair or other fibres in the backing coat mixes will greatly aid the minimising of shrinkage cracks and is strongly recommended.

Second Coat
Before the application of the second coat, the first coat should be left for a minimum of 4 days, during which time the first coat should be checked for shrinkage cracks and also kept damp to avoid rapid drying out. Before applying the second coat the first coat should be damped down, making sure the water is absorbed into the render and not sitting on the surface. The second coat is applied using the same tools as the previous coat.

The coat thickness should be 8mm and no thicker, the coat is applied left to stiffen and then scoured up to the required finish, as before over scouring should be avoided. Good curing once the work has been completed is essential if shrinkage cracks are to be minimised.

After the application of lime renders, controlled curing and protection will be needed to ensure maximum strength and durability are achieved. The lime putty will stiffen initially due to absorption of moisture into the wall and evaporation to the air and will cure and harden as a result of Carbonation’, (the re-absorption of carbon dioxide).

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The NHL 2 will set more quickly as a result of a chemical reaction with the clay content known as the ‘hydraulic set’. ‘Carbonation’, (the re-absorption of carbon dioxide), also takes place. This process is best achieved in warm and moist conditions, which allows the new works to dry slowly. Therefore, during and after completion of the work, it is essential to ensure ambient conditions.

Rapid drying by the sun, wind or artificial heat will all have a detrimental effect on the final outcome of the lime finishes. Temperatures below 5ºc will slow the carbonation and hydraulic setting process and frost conditions will damage un-carbonated areas, through the action of freeze-thaw (expansion/contraction) resulting in feeble and crumbly finishes.

Excessive shrinkage is a result of rapid drying, and this can lead to separation between coats and background and cracking. Rapid drying of the surface of new mortars, can also lead to the pores of the mortar becoming blocked with fine material, transported to the surface by the passage of water evaporation too quickly from the mix, this will inhibit the carbonation process taking place deeper into the new mortar.

The best way to control and protect the carbonation process is to form a microclimate for the new work. Where the new work is scaffold, this can be a reasonably simple job. Scaffold netting is very useful for reducing the effects of wind. In addition to this in warm or hot conditions, damp hessian can be placed against the new work and then covered by sheeting to stop rapid drying. New work should be damped down for a minimum period of 10 days after completion and longer if possible. The emphasis should be on damping down as opposed to saturating new work. Provision should be made for damping down over weekends, holidays etc. In cold weather, the work must be protected from frost attack, by using thermal blankets e.g. polystyrene sheets. Hydraulic plasters/mortars will stand up to cold conditions after 3-4 weeks of hardening. It should be remembered that prolonged periods of cold temperature will slow the overall hardening process and extended periods of protection will be called for.

Any cracking occurring after the first few days of application can be remedied by scouring the surface with the wood float around the area of cracking to fill and compact the crack.

Health and Safety

Harling by its nature of application carries a risk factor, and therefore personnel should wear protective equipment, particular attention to be given to EYE AND SKIN PROTECTION. Eye wash should always be on hand. Gloves should be worn when working with any mortars or plasters.

LIME PLASTERING

Preparation of the Wall Surface

The successful application, bonding and correct hardening of hydraulic lime mortars, requires that the background should be clean, free from vegetation, free of containments and reasonably dry throughout the wall mass. The wall should be structurally sound and the masonry and bedding mortars in good condition.

The application of various dubbing out coats should not be seen as remedial repair work to the masonry or brickwork background.  The replacement of missing or damaged masonry or open joints should be repaired before any plastering of dubbing out coats are applied. Where excessive hollow or unevenness is present the areas should be corrected by use of mortar and stone or brick slips bedded into the mortar, with the aim of presenting a reasonable flat background before plastering commences.

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Suction Control and Bonding

Before the application of any new lime coatings, hydraulic or non-hydraulic, it is vitally important to check to the degree of suction within the background.  Poor or excessive suction can result in a weak bonding with the substrate caused by rapid drying of the newly applied render, which will result and a weak and powdery interface which will lead to later failure and separation. Where there is little or no suction further action will be required to help bond the coating to the substrate.

In situations where suction needs to be controlled, wetting down will be required, on dense blocks or near impervious masonry.  Simply dampening the surface with a mist spray may be all that is required, but on very porous surfaces such as old brickwork considerable wetting will be required. Wetting the wall by use of a hose, working from the top of the structure, downwards, may need to be carried out the previous day or several times throughout the day before rendering commences.

The objective of the suction control is to achieve a thoroughly damp surface, but not wet, i.e., the surface must not have running or standing water remaining on the masonry or brick, this will form a barrier between the coating and substrate, also lime mortars adhere and stiffen through a certain amount of suction.

On dense or near impervious background, it may be necessary to apply a sand/lime splatter dash coat to the background to act as a mechanical key.

Salt Contamination

Where new lime coatings are to be applied to masonry which is salt contaminated, the masonry should be allowed to dry fully before applying new renders. This will allow salt to be detected on the masonry and mortar joint surfaces, if excessive salt is identified clay or lime mortar poulticing may be required.

Specialist advice should be sought, if the technique is considered where salt is detected on the mortar joints, rake out the joints to a depth of 50mm, as this is likely to be heavily contaminated and in a weakened condition and re point (See re pointing). Salt contaminates should never be washed from the surface, as this will result in the crystallized salt returning to a soluble state and retreating back into the pores of the masonry or brick. Where detected on the masonry surface, the salts should be brushed from the surface and cleaned away from the structure

Techniques of Hydraulic Lime Plastering

The techniques employed in the application of hydraulic lime plasters should be to ensure a correct bonding with the background while striving to minimise shrinkage and rapid drying.  These techniques should be followed throughout the plastering process.

Lime plastering is generally applied in 3 coats, but it is common to find 2 coats or even single coat work in vernacular or early structures.

In 3 coat work the first coat on masonry or brickwork is generally known as the scratch coat or render coat.  This coat is applied in a coat of approximately 10mm thickness. It can be applied by use of a steel trowel or thrown onto the wall by use of a harling trowel and then flattened in by the steel trowel. When this coat has firmed up but has not gone hard, the plaster is keyed or scratched up to produce a key for the following coats. The keying up is carried out by use of a lath scratcher or similar toothed instrument and care should be taken not to cut through the plaster coat back to the background. The keying up is generally in the shape of diamonds of approximately 30mm spacing. This coat should be allowed to harden for 72 hours minimum before further coats of plaster are applied.  Before applying the second coat, the first coat should be checked for shrinkage cracks. These should be filled with plaster before proceeding with further coats. The first coat should also be brushed down to remove any dust which may have blown onto the surface. The first coat should then be damped down to ensure that the second coat is applied to a damp but not wet surface.

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This second coat is called the floating coat and is the coat which is straightened to ensure a flat and even surface.  After this coat has been straightened, the surface of this coat is scoured up with a timber or polyurethane float. The purpose of the scouring is to compact the plaster and counteract shrinkage. This process is generally required to be carried out at least twice.  During the scouring up, any small holes should be filled before the finishing coat is applied. This coat should also be allowed to dry and harden for 72 hours before applying the final coat.

After once again damping down the floating coat, the final coat is applied. This coat known as the setting coat, is applied in two layers, coat on coat, with the trowel. This coat is applied in a way similar to skimming. This material should be of a wet enough consistency to allow a long and even spread. Once the setting has been applied, it is scoured in a similar manor as the floating, to counteract shrinkage and then the surface can be left with a rubbed up rendered type finish or polished smooth with a steel trowel

General Mix Guide

Backing Coats: 1st and 2nd, 1 part NHL2 or NHL3.5 to 2 parts or 2 and a half parts SHARP WASHED SAND. The use of fibre or HAIR in these coats is recommended. Finish Coat: 1 part NHL2 1 part Fine Silica Sand Note Highly polished surfaces will not allow the passage of moisture as well as an open textured surface.

Curing Lime Based Renders & Mortars

A lime binder must be cured once it has been placed because it requires time to fully hydrate before it acquires strength and hardness. Curing is the process of keeping the mortar/render under a specific environmental condition until hydration is relatively complete.

Good curing is typically considered to use a moist environment which promotes hydration. Increased hydration lowers permeability and increases strength, resulting in a higher quality material.  Allowing the mortar/render/concrete surface to dry out excessively can result in tensile stresses. The still-hydrating interior cannot withstand these stresses, causing the mortar/render/concrete to crack.

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Protecting Lime Renders

Standard practice for protecting lime renders would be in the form of Hessian sheeting draped over the scaffold in relatively close proximity to the render. This should be left in place for at least a week. This is a standard requirement for any kind of rendering and one that is regrettably seldom practiced. Once again it’s very important for the hydration/hardening phase of the binder.

Cornish Lime stock four grades of Hessian; 229, 273, 320 & 360 GSM (Grammes per Square) Metre where the weight is relevant to the weave and amount of fabric used per square metre. The most commonly used for curing is the 229 & 273 GSM, the heavier fabric is more generally used for frost protection.

When appropriate we would advise the addition of a proprietary polypropylene or fiberglass reinforcing fibres added to the mix as an aid to control shrinkage cracking in the base coats.

Issues

1, Drying Too Quickly

Where a render is allowed to dry out too quickly hydration and carbonation of the binder is inhibited, resulting in drying shrinkage. There are two principle types of drying problems both of which will be manifested as cracks. The first, plastic shrinkage is the consequence of the rapid evaporation of mixing water from the mix (while in its plastic state). This leads to increased tensile stresses at a time when it has not gained sufficient strength. Plastic shrinkage cracks will be manifested in the first 48 hours.

The second, drying shrinkage is from the effects of climatic conditions such as wind and high temperatures or exposure to strong sunlight (compounded during times of low humidity).  Cracking from drying shrinkage tend to take that much longer to manifest but the outcome is much the same. Another consequence of rapid drying is that the mortar may become friable.

2, Excessive Water

The consequences of too much water in a mix can compound the plastic shrinkage, which as previously mentioned is likely to be manifested in the first few days following application. Water in a mix takes up volume and is given up during the hydration process

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3, Moving On Too Quickly

The consequences of applying subsequent coats of render coats to soon onto the previous coat may result in stress cracking as a result of unequal contraction (differential drying) between the two layers. We would advise that the backing coat should be allowed to achieve a sufficient set prior to applying additional coats.

4, Thick Top Coats

The application of excessively thick top coats can result in stress fracturing in the coat as a result of unequal compaction when finishing the render coat.  The purpose of floating (rubbing up) is primarily decorative, however it performs a technical function in that by closing the surface will help reduce the Ingress of water. Also where a top coat is too thick it will be extremely difficult, often impossible, to compress the whole thickness to an adequate level.

As well as supplying Hessian, Cornish Lime also supply a Wintermix product as part of our Cornerstone range. Please Contact Us for further information on this product.

Adding Hair or Fibres to Lime Mortar

Hair or Fibres have traditionally been used as reinforcement in lime renders as a means of improving tensile strength and to reduce shrinkage cracking. Introduced into the mix by teasing the hair into the mix prior to the mortars use. This is a time-consuming job as it involves teasing the hair into the mix as it is mixed. Clumps of hair simply cannot be placed into the mixer as they will remain in the mix as large balls of hair and will not part no matter what type or method of mixing is used.

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The use of hair in renders is most definitely advised on walls with a lath background. Hair should be added to the first two coats but not the final setting coat. As for its use on a solid masonry background this is somewhat questionable (in our opinion) but does act as a good reinforcement by reducing the amount of shrinkage cracking in thick coats of render.

Cornish Lime supply a range of types of hair along with polypropylene / flock fibres and can give guidance on the quantities needed. We also supply CLM28F, a ready-mixed lime putty mortar with the addition of fibres included.

Fibres Vs Animal Hair
Fibres work nearly as well as natural animal hair but are cheaper and far easier to introduce into the mix. Rather than teasing in as for hair, simply sprinkle the fibres into the mix. This then requires only a few minutes mixing for them to be thoroughly distributed.

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The hair used would be any animal hair, rough in texture from goats, cows, horses (body hair not the mane or tail) amongst others. Evidence of other types of reinforcement has been found which include hemp or jute fibres among others, but the use of hair is fairly universal for this role.

Hair is indeed ‘the’ traditional and often preferred choice with fibres as a very modern yet ‘honest’ approach to a centuries-old practice.

How to Apply a Three Coat Lime Render

A guide by Cornish Lime, using NHL or Lime Putty

There are a number of different substrates you could be working with, from a simple masonry wall to a timber lath substrate, and we have tried to keep the following guide as generic as possible.

The following guide applies equally to both NHL and Lime Putty renders, and Cornish Lime stock an extensive range of ready-mixed base coat and top coat renders to suit all applications, supplied as both NHL drymix and lime putty. We also supply premium quality lime putty plasters for fine plastering work.

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Preparation is key

As with most things in life sufficient preparation is key, and when carrying out any type of rendering making sure the surface is thoroughly cleaned and free of all dust or debris is of paramount importance.

Also ensure the surface is not too smooth and, if so, first score or roughen the surface sufficiently to provide a good key for the first coat to adhere to.

Avoiding the pitfalls of lime rendering

Lime renders can be temperamental and do require due care during their application and their infancy, and can fail from excess shrinkage, drying back too quickly, or weather damage during the early stages of their set.

However, applied properly, they will provide both protection and decoration to virtually any structure.

Failure can usually be avoided through basic preparation and, when necessary, sheltering from poor weather. Simple wetting tests, observation and planning at the outset is also strongly recommended.

If in any doubt contact us.

Solutions to common problems

Shrinkage – as initial shrinkage takes place in the drying out phase, this can be beaten back by using a plasterers’ float and dampening the wall as required – pressing the float home evenly, and in a close circular motion but only if necessary.

Drying out too quickly – Lime renders should never be allowed to dry too quickly, and a render that is simply allowed to dry out too quickly is more than likely to fail.

There is a vast difference between a render that has been allowed to carbonate and one that was simply allowed to “dry out too quickly”.

Pre-wetting the surface

To better control potential shrinkage, we highly recommend pre-wetting the surface to avoid moisture being drawn out of the render coat and into the substrate. Try to avoid over-wetting – pump-up garden sprayers are well suited for this purpose, as a hosepipe will deliver too much water in most cases. In the case of very porous materials such as cob, chalks, and clunch, along with different types of soft brick or stone, the use of a hosepipe may indeed be appropriate.

Weather permitting

You should also pay attention to the weather, as strong sun, wind, frost and rain will all have a bearing on the overall performance of a long-lasting, defect-free lime render.

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Work needs to be kept dry enough to allow the lime enough time to set, but do not allow it too dry back too quickly. Try to shield work from direct or wind-driven rain, and where necessary use hessian curtains to stop the work drying out too quickly from wind or strong sun. It is also very important to avoid frosty conditions during the render’s early set, particularly within the first 14 days.

Filling large voids

As lime mortar is more expensive than the stone usually to hand, you can pack out large voids or hollows with a combination of lime mortar and stone.

Scoring / Scratching in

Once the INITIAL set has taken place, key the wall using a convenient tool to make a groove in the render of sufficient depth that will allow the subsequent coat something to grab, or hang on to, without over scoring or tearing the backing coat.

Diamond keying is recommended for scratching in, and a three-pronged lath scratcher is a simple tool to knock up.

HOW TO LIME PLASTER ONTO LATHS

Lath and plaster is a building technique used mainly for interior walls in period buildings. A lath is a narrow strip of wood approximately 2 inches wide which is nailed horizontally to each stud in the frame. Each lath is spaced approximately 1/4inch ( thickness of the average little finger ) away from the next leaving enough space for the mortar to push through and hook over the lath to form a key laths.

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Preparation

The LATHS must be free from dirt, grease and vegetation. These elements should be removed before damaged laths are repaired. Spray a clean cold water mist on to the laths prior to applying the scratch coat.

First Coat

Using a ratio of 1 part LIME PUTTY to 2 and a half or 3 parts SHARP WASHED SAND with evenly distributed HORSE HAIR throughout the mortar and working to a layer of 10mm thick, push the plaster into and across the pre wet laths at a 45 degree angle to the laths.The plaster should be left until set hard but protected from heat and draughts.

Second Coat

The second coat also known as the intermediate or float coat will be applied similar to the first coat only without any hair being added to the mortar. You must ensure you wet down the first coat with a fine mist before spreading a 7mm coat, before scratching up and leaving in preparation of the finish coat.

Finish Coat

For the final top coat use a fine mix of 1 part LIME PUTTY to 2 parts well graded super FINE SAND. Remembering to wet down the surface before application with a fine mist spray. Allow time for the water to absorb into the plaster then apply a coat of approximately 5mm. Rule off and leave for a couple of hours. When ready rub up the finish coat using a plastic or wooden trowel to bring back the fat until the required finish has been reached.  In the case of any light crazing this can be rubbed up and trowelled out.

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Health and Safety

Lime mortar by its nature of application carries a risk factor, and therefore personnel should wear protective equipment, particular attention to be given to eye and skin protection. EYEWASH should always be on hand. GLOVES should be worn when working with any mortars or plasters.