Limewash (or other breathable paint finish) is the final part of the protective envelope for structures coated with a lime render either smooth or rough as in the case of harling.
- It provides a protective, permeable and renewable skin of calcium carbonate.
- The best limewash is manufactured from mature lime putty.
- Surfaces to be limewashed must be clean and free from grease. The best results are achieved on porous backgrounds i.e. lime plasters and renders. Surfaces and joints should be brushed free of loose dirt with a natural bristle brush (not a wire brush) to ensure a good bond, and open joints flushed out with water. Previously limewashed surfaces must also be well brushed down and any loose limewash scraped off.
- Any mould should be treated with fungicide and thoroughly washed off with clean water. Do not use fungicides which contain silicon. All lichens, moss and vegetation should be removed using a compatible biocide and bristle brush, or a steam cleaner. All forms of biological growth will retain moisture and can grow back through the new coating.
- The area to be limewashed needs to be dampened down with clean water prior to application. Dry porous sandstone will normally have excessive suction. Dampening down, to control suction between the background and newly applied materials is essential. Applying limewash to a dry surface will cause excessive dusting and an unpleasant white bloom on the surface. This is due to rapid drying and the lime being brought to the surface. Spray about 3sq/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.
- Denser, more impervious stones, e.g. whinstone or other metamorphic stones, or some granite have very little natural suction, and lime mortars may not adhere to these stones if there is any surface water present. These low-suction backgrounds need no dampening down.
- Limewash is best applied using a 5 or 6 inch brush working the wash in thoroughly over the surface. It is important to stir the limewash before and regularly during the application.
- Building up a thickness of limewash using many light coats is the most desirable approach. Apply, working the wash well into the surface. 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.
- A period of 24 hours should be given between coats, to allow the wash to dry. It is important to rewet the previous coat before applying the next coat.
- At least 5 – 6 coats are needed on new work, up to 10 can be added if time allows, building up a satisfactory depth of colour.
- For best results lightly buff up the surface when it has “taken up” a little using a dry worn brush.
- As with all operations using lime products, protection will need to be provided before, during and after application. It is very important that there is no danger of frost for several weeks after the limewash application.
- Limewash should be maintained with further coats every 2 – 4 years depending on exposure.
- The application of limewash is a very important part of the structures protective skin yet many specifications either omit it, or not enough resources are channelled towards it.
Limewash serves a very functional purpose yet it is also decorative. Plain un-coloured limewash will take on the colour of the lime used; this can range from pure white through to grey or buff coloured. Generally earth pigments were used to colour the limewashes, most commonly ochres, 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. When using pigmented limewashes, it is advisable to calculate the total amount and to mix this in one batch. Slight variations in colour are possible between batches.