The finish is the thinnest layer in the wall and the one most able to ruin it. A breathable build-up painted with an ordinary plastic emulsion is a sealed wall, and it took one afternoon to make it one.
Six questions, and the answer to the first usually decides the rest.
Hydraulic lime render, two or three coatsGoes straight onto the surface with no board or mesh, because the background is soft, keyed and vapour-open already. Wait until the wall is genuinely dry, not merely firm.
Lime putty skim, or clay plasterBoth stay open. Clay is the easiest to live with — no caustic handling, forgiving to apply, and it buffers humidity better than anything else on this list.
Lime, and nothing harder than the wallThe rule for old buildings is that each layer should be softer and more permeable than the one behind it, so damage happens in the finish where it is cheap to repair.
Tadelakt, or tiles on a lime backgroundLime handles humidity well but not standing water. Polished lime plaster is genuinely waterproof where it matters; a plastic paint in a bathroom traps the humidity behind it instead.
Find out what it is firstA breathable paint over an old plastic one is just a breathable-looking wall — the seal is the layer underneath. Either strip it, or accept that this wall is not breathing and stop paying for finishes that assume it is.
Hydraulic lime render, or lime-hemp for the extra insulationUse a more hydraulic lime the wetter the exposure. Keep it off the ground — render bridging into soil wicks water straight up.
Standard vinyl and acrylic emulsions form a plastic film. That film is close to vapour-tight, and one coat of it on the inside face is enough to stop a breathable wall breathing — a wall, a roof or a hempcrete cast that were all designed around moisture being able to leave. It is the cheapest layer in the build-up and the one most likely to waste the rest. Silicate, clay and limewash paints all stay open; ordinary emulsion does not.
| Finish | Coats | Between coats | Fully cured |
|---|---|---|---|
| Lime putty skim | 2 | 1–2 days | Weeks |
| Hydraulic lime render | 2–3 | 3–7 days | Months |
| Clay plaster | 2 | 1 day | Days |
| Lime-hemp render | 2 | 5–7 days | Months |
| Limewash | 3–4 | 1 day | Weeks |
| Silicate paint | 2 | Hours | Days |
Covered above, and worth repeating because it is done last, often by somebody who was not there for the rest of the job and was handed a tin. Write it on the wall if you have to.
Onto a background that is still wet, the finish cannot carbonate properly and traps water behind it. Every one of these materials rewards waiting and punishes hurry.
Added “to help it go off”. It makes the coat harder than its background, so it cracks and lets water in behind itself — and then holds it there. On any soft or historic background this is the classic damage.
Lime dries too fast and crazes. Shade the wall, close the windows on that side, and mist it if the weather is fighting you.
Both lime and clay take mineral pigments, and clay comes in a wide range of body colours as it is. Colours are softer and more variable than a mixed emulsion — the surface shifts through the day, which is either the point or the problem depending on the person.
It is caustic. Goggles and gloves are not optional, and lime in an eye is an emergency rather than an irritation. Clay plaster has none of this, which is a real reason to choose it indoors.
Wet a coin-sized patch and leave it a minute. Limewash and clay soften or come away on a finger; a plastic paint sits there unchanged and can often be peeled in a sheet from an edge.
Less well than filler and emulsion, and it is honest about movement. A lime wall shows the building living, which is usually preferable to a rigid finish that hides it until it fails all at once.