What is limewash used for?
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.
In its simplest form, limewash is lime putty diluted by water to turn it into a milky consistency.
Additives for Limewash
How long does limewash last?
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 unaffected 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 un-coloured limewash will take on the colour of the lime used; this can range from pure white through to gray or buff coloured.
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.
How do you apply limewash?
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.
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.
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.
How many coats of limewash do I need?
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.
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 when applying limewash 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.
IT IS VERY IMPORTANT THAT THERE IS NO DANGER OF FROST FOR SEVERAL WEEKS AFTER THE LIMEWASH HAS BEEN APPLIED.