Where this starts
Paint coming away from newly plastered walls is one of the most common decorating failures, and it is usually a preparation problem rather than a paint problem — which is worth establishing early, because the instinct is to buy a better paint and repeat the failure.
New plaster presents two difficulties at once. It is highly ABSORBENT while it is still curing and drying, so a normal emulsion applied straight onto it has its water drawn out before the film can form properly. And a trowelled finish can be dense and closed, with very little surface texture for a coating to key into. A thinned first coat — a mist coat — addresses both by soaking in and giving the topcoat something to hold.
Moisture is the other family of causes, and it separates cleanly from the first by TIMING and by what comes away with the paint. A bond failure shows up on the first or second coat and peels cleanly, leaving sound plaster behind. A moisture problem appears later, often in a patch rather than a whole wall, and the plaster behind it is damp, soft, or salted.
The list below is ordered by how often each turns out to be the answer on a new plaster surface. None of them is a verdict about your wall: each is separated from the others by something you can look at.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
No mist coat, or a topcoat used as one
Most commonThe paint peels in large clean sheets or strips and the plaster behind it is sound, dry and often shiny. The back of the peeled film is smooth rather than carrying plaster with it.
How to check: Ask what went on first. A full-strength emulsion, a vinyl matt, or a one-coat paint straight onto bare plaster is the answer on its own.
Painted before the plaster had dried
CommonPatchy adhesion rather than uniform, often worse on external walls and in corners, sometimes with a visible darker area. The plaster may still be a deeper colour where it is damp.
How to check: A moisture meter reading on an affected area against an unaffected one, or simply the dates: plaster needs weeks rather than days, and longer in cold or still conditions.
A sealer or PVA used as a primer
CommonThe paint and a glossy film come away together, and the back of the peeled piece is shiny. The surface underneath may feel slick.
How to check: Look at the back of a peeled flake against the light. A PVA layer leaves a distinct film rather than plaster dust.
Dust or plaster residue left on the surface
Less commonThe peeled film carries a fine layer of powder on its back, and the failure is patchy in a way that follows where the wall was rubbed down.
How to check: Wipe a sound area with a dark cloth. Heavy dust transfer means the surface was not cleaned before coating.
Damp reaching the plaster from behind
Less commonThe failure is confined to a patch rather than the wall, often low down, near a wall-to-floor junction, or below a window. Salts may be visible as a white bloom, and the plaster is soft.
How to check: Compare a moisture reading in the affected patch with one a metre away on the same wall. A local high reading points behind the plaster rather than at the coating.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It separates the whole first group of causes in one observation: clean and smooth means a bond failure, powder means dust, a shiny film means a sealer, and plaster stuck to it means the plaster itself has failed rather than the paint.
- 2
The commonest cause is a missing or wrongly made mist coat, and it is settled by a question rather than a test. The date the plaster went on, against the date it was painted, settles the second.
- 3
It separates drying-out moisture, which is uniform and improves with time, from a damp source behind the wall, which is local and does not.
- 4
A whole wall or several walls points at preparation. A patch, particularly low down or beneath an opening, points at water getting in.
- 5
A newly plastered room that has been shut up, heated and used produces a lot of condensation on the coldest surfaces, which can hold a wall damp long after the plaster would otherwise have dried.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where a moisture reading stays high in one area after weeks, or rises — that is water arriving from somewhere, and finding the source matters more than the decoration.
- Where a white crystalline bloom keeps returning after it is brushed off, which indicates salts being carried out of the wall by moisture rather than a surface problem.
- Where the plaster itself is soft, hollow-sounding when tapped, or comes away with the paint — the substrate has failed and recoating will not fix it.
- Where the affected wall is below ground, or where the damp follows a line rather than a patch, both of which point at the building rather than the finish.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.