Before the plaster dries

Lime plaster is the only finish on our projects that cannot be corrected later. Once it has gone off, the decisions in it are permanent. This is the note we give every site team before the first coat goes on.

The window is narrower than you think

From the moment the finish coat is applied there are somewhere between forty minutes and three hours in which the surface can still be worked. The range is that wide because it depends on the suction of the background, the humidity, and how much sun is coming through the windows that day.

This is why we do not schedule plastering around the trades. We schedule the trades around the plastering. If the finish coat is going on Tuesday, nothing else happens in that room on Tuesday, and the windows are dressed the evening before so the light is consistent.

A room plastered in strong afternoon sun and finished in shade will have two different sheens on two walls, and no amount of later work will bring them together.

What we check at the scratch coat

The finish coat is only as good as what is under it, and the scratch coat is where problems become permanent. We walk the room before the finish goes on and check four things.

Flatness, with a two-metre straightedge, in both directions, on every wall. Tolerance is three millimetres. A hollow that reads as nothing under a straightedge will read as a shadow under a wall light for the next thirty years.

Suction, by wetting a patch and watching. If the water vanishes in seconds the background is too thirsty and will pull the water out of the finish coat before it can be burnished. That background gets damped down, sometimes twice, before anyone opens a bag.

Then the arrises, which want to be straight and consistent — we run a bead where the corner is exposed and a hand-formed arris where it is not, and the two must not appear in the same sightline. And finally the junctions with joinery, because plaster meeting a shadow gap is a detail and plaster meeting a skirting is a compromise.

Burnishing, and the argument about sheen

Burnishing is what makes lime plaster look like lime plaster: the trowel is passed back over the surface as it goes off, compressing the top layer and drawing a faint polish out of the aggregate.

How much you burnish is the single biggest aesthetic decision in the room, and it happens in a ninety-minute window with a trowel in someone else’s hand. That is why we agree it on a sample board, on site, with the same plasterer who will do the room, weeks in advance.

Our default is what our plasterer calls a half burnish: worked enough that the surface catches light at a grazing angle, not so much that it reads as polished stone. Full burnish belongs in bathrooms, where it earns its keep by shedding water. In a living room it looks like money and nothing else.

Curing is not drying

The most common failure we see on other people’s sites is a lime finish that has dried rather than cured, and the difference matters.

Lime hardens by taking carbon dioxide back out of the air, slowly, over weeks. If the surface dries out before that reaction has run, you get a plaster that is powdery to the touch and will keep dusting for years. Warm dry air, which is exactly what a site heater produces, is the enemy.

So: no heaters in a freshly plastered room for at least a week. Windows cracked, not open. If the weather is hot, the walls get misted twice a day for the first three days. It is unglamorous and someone has to be paid to do it, and it is the difference between a wall that survives and a wall that chalks onto a sofa.

The handover note

Every client with lime plaster gets one page at handover. It says: do not wash it for three months. Do not hang anything heavy without asking. Small marks will burnish out with a dry cloth and pressure. And a scuff that will not come out can be repaired invisibly by the same plasterer, because lime repairs into lime in a way that paint never repairs into paint.

That last point is the whole argument for the material. It is the only wall finish we use that gets easier to look after as it ages.