Fresh plaster always looks dry long before it actually is. Paint it too soon and you trap moisture behind the film, which leads to peeling, flaking and patchy colour. Here is what really happens as plaster cures and how to time that first coat properly.
As a rough guide, allow 4 to 6 days for a skim coat over plasterboard, and longer for backing plaster applied to brick or block. A full re-plaster, where bonding or hardwall has gone on before the skim, can need anywhere from 7 to 14 days before it is genuinely ready to paint.
Those ranges are not fixed. A small patch repair in a warm, ventilated room can be sound in under a week, while a freshly plastered Victorian terrace wall in a cold back room around Liverpool in winter can easily take a fortnight or more.
The colour change is your best signal. Wet plaster is a deep, even chocolate brown. As it cures it turns a consistent pale pink with no dark patches left, especially around sockets, corners and where the wall meets the ceiling, which are always the last areas to go.
If you can still see darker shading anywhere, it is holding moisture and is not ready. Touch tells you little because the surface dries first, so trust the colour across the whole wall rather than a single spot.
Standard emulsion forms a film on the surface. Put that over plaster that is still releasing moisture and the water has nowhere to escape, so the paint blisters, peels or dries to a flat, blotchy finish that no amount of recoating will fix.
It also undoes good plastering. A smooth, polished finish is wasted if the decoration fails within weeks, and stripping failed paint off fresh plaster risks pulling the surface with it.
Ventilation does far more than heat. Open windows and let air move through the room, and use a dehumidifier if you have one rather than blasting a heater straight at the wall, which dries the surface too fast and can craze it.
Gentle background warmth from normal central heating is fine. Avoid putting furniture tight against a new wall while it dries, as that traps moisture in exactly the spots you want airing.
Once the plaster is fully pale and even, the first coat should be a mist coat, which is watered down emulsion, typically around three or four parts paint to one part water depending on the brand. This soaks in and seals the surface so your top coats sit properly.
Skip the mist coat and your finish coats are far more likely to flake later. After it dries you can apply two normal coats, and the wall will look its best for years.