Traditional and Mineral Paints Over Older Wall Finishes

A potassium silicate binder reacts with the surface and effectively becomes part of it, which requires a mineral, absorbent substrate and rules out an existing film-forming paint. Limewash is slaked lime in water that carbonates and stays vapour open. Soft distemper is whiting bound with animal glue size, deliberately weak. Clay paint is fine clays and mineral fillers with a small binder proportion. Emulsion is a polymer dispersion that coalesces into a film sitting on the surface.

Read as a set, that is five different answers to the question of how a coating stays on a wall, and the answer determines what each one can be applied over and what can later be applied over it.

Lime and cement-based plasters and renders, masonry and previously mineral-coated surfaces are the usual candidates. An existing film-forming paint is not, because the coating cannot bond into a surface that is already sealed. Timber, plastics and most manufactured boards are outside the mechanism entirely.

There is a further distinction worth confirming: many products are dispersion silicates carrying a small organic addition to widen what they can go over, and pure and dispersion products differ in what they may be applied to. The data sheet is the only reliable guide, and the type used should be recorded.

Emulsion lays a continuous film over the surface and holds moisture behind it if the wall is damp. Limewash forms no such film, stays vapour open, and at the end of its life erodes, dusts and weathers back unevenly instead of blistering and lifting in sheets, so redecoration means washing down and recoating rather than stripping.

Neither is a better paint. They behave differently and they belong on different walls, and a permeable decoration cannot make an impermeable wall breathe however often it is asked to. A limewash colour also lightens as it dries and keeps shifting as it carbonates, so a wet wall is a poor guide to the finished shade.

Soft distemper is regularly assumed to be an old word for emulsion. It is not washable, it marks readily, and it can be softened and taken off again with water. That reversibility is the whole reason it survives: successive schemes can be removed rather than accumulated, so cornices, ceiling roses and modelled plaster stay crisp instead of slowly clogging.

Modern film-forming paint applied over soft distemper is a recognised cause of adhesion failure, and it is discovered late. Two related materials share the name and behave differently, so establishing which of them is on a ceiling is the first step rather than an optional one.

Clay plaster is a trowelled body coat with real depth: it levels the wall, takes texture and contributes material to the room surface. Clay paint is a thin decorative coating with no levelling ability at all, applied over a wall something else already made flat. The trade names overlap and the two are routinely confused.

Formulations also vary a great deal between products sold under the name, from almost purely mineral to substantially bound with synthetic additions, which makes the specific documentation more useful than the category. A deeply matt finish also makes wall flatness more visible rather than less.

Pigments have to survive strongly alkaline binders, which pulls the palettes towards earth and mineral colours and narrows the range at the deeper and brighter end. Colour develops as a lime coating dries and carbonates. Distemper's whiting gives a soft warm white rather than a modern brilliant white, and the contrast at a junction with new work is noticeable.

These are not defects. They are the reason the finishes look the way they do, and they are best judged on a sample area on the actual wall, in the light that room really gets, rather than from a card or a wet surface.

Silicate material is alkaline in use and can etch glass, metal and adjoining surfaces, so masking and protection are a far larger part of the job than with ordinary paint. Limewash is soft when dry and transfers onto clothing in narrow spaces. Distemper is not washable at all, and occupiers reasonably assume any painted ceiling can be wiped.

That last point is the one most often skipped at handover. A note for future occupiers explaining what a surface can and cannot tolerate is the difference between a finish that survives and one that is ruined by an ordinary redecoration.