Technical Guidance TGN 2. (RJ 3.04)
Limewash is a traditional permeable paint for use on porous building materials such as lime plaster, lime render, limestone and soft brick. It is a thin dilution of non-hydraulic lime, water, and alkali-fast powder pigments. Exterior grade limewash contains biocides and additional binders – casein, tallow or linseed oil – to improve ‘rain-shedding’ and durability.
Pure limewash is normally reserved for internal use. Casein limewash is an exterior grade that is often also used internally as it is less likely to rub off, and Tallow limewash is an exterior grade rarely used internally.
Limewash can be applied to newly applied lime plaster and render before it has fully carbonated as the permeable limewash does not impede the carbonation of the plaster beneath.
Limewash, due to the thin consistency, can be ‘messy’ to apply and protections must be provided. Special care must be taken if oak is present as it is permanently stained by lime. Lime is alkali and can cause serious eye damage and suitable protective clothing and full-face protection must be worn. Applying limewash to ceilings is particularly hazardous, and soft distemper is often preferred for the decoration of ceilings. Indeed, distempers are often used in preference to limewash for the internal decoration of walls and ceilings.
Limewash can only be applied to previously painted surfaces if the existing paint is limewash, and should not be applied to previously distemper or emulsion painted surfaces.
Limewashes require a surface that is firm, clean, and porous. Remove all loose material and dirt and previously limewashed surfaces should be thoroughly soft-brushed and washed with a wet sponge to remove dust, dirt and all loose chalky material that is removed readily.
Any algae or mould must be treated with an algaecide or fungicide and thoroughly washed off with clean water. Do not use proprietary fungicides containing water repellents. Make good and fill with a compatible lime filler.
Limewash must be applied to a damp surface. Lime plaster and render, porous stone and brick must be thoroughly moistened with a mist spray prior to application.
Ensure limewash is within shelf-life period. Do not use in extremes of heat and cold.
Protective clothing and full face protection must be worn. Limewash must be thoroughly mixed, preferably with a mechanical whisk, to ensure even distribution of pigments and ingredients immediately before and during use as it readily settles out.
Externally, adequate protections from sun, wind, rain and frost must be provided, and surfaces must be damp. Porous pre-wetted surfaces often need additional mist-spraying during application to ensure that the limewash does not dry too quickly.
Apply the limewash to the damp surface using a brush, not a roller, working it into any cracks. It is applied in thin coats - limewash appears transparent on application and care must be taken to avoid thick build-up, which will craze on drying out and may obscure detail.
Normally, 3 coats are required, occasionally more. Each coat must be protected from drying too quickly by regular mist-spraying, and a reasonable time (normally at least 24 hours) allowed between coats. It is normal to mist-spray the previous coat prior to the application of the next coat.
RJ 3.01 Pure Limewash
RJ 3.02 Casein Limewash
RJ 3.03 Tallow Limewash