Can I skim over a painted wall, or does the paint have to come off?

Plaster does not stick to paint; it sticks to whatever a bonding agent can key into, and the paint becomes the weakest layer in the sandwich. So the whole decision is about how well the existing paint is stuck to the wall, and how much of the wall is like that.

Safety

What you'll need

• a grit-filled bonding primer
• a scraper and a stiff brush
• a wide taping knife or trowel to test with (optional)
• masking tape, for the adhesion test

Step by step

1. Identify the paint before anything else.Matt emulsion, vinyl silk, eggshell, gloss, or an old distemper. Rub a damp cloth on it: distemper comes off as a chalky bloom and is the one absolute no.
2. Test the adhesion.Press strong tape hard onto the wall in several places and rip it off. Paint coming away with it means the paint is not securely bonded, and anything you plaster onto it will come off with it.
3. Scrape and check the whole wall, not one patch.Run a scraper firmly across the surface. Any area that lifts, flakes or sounds hollow is an area that will fail. Map it before deciding.
4. Rule out the definite no-gos.Glossy and silk paints have no absorbency and a slick surface. Flaking paint has no bond. Distemper is water soluble. Textured coatings, especially pre-2000, must not be disturbed at all until assessed for asbestos.
5. For a sound matt emulsion, key the surface.Abrade it to break the sheen and give the primer something to grip, using wet methods or an extracted sander — and not at all if there is any chance of lead paint.
6. Apply a grit-filled bonding primer, not PVA.The grit is the point: it provides the mechanical key that a painted surface lacks. Let it cure as the maker specifies before plastering.
7. Skim in thin coats and expect to work fast.A primed painted wall has almost no suction, so the plaster stays wet longer and behaves differently from bare board. It is a harder wall to skim than plasterboard, not an easier one.
8. If a lot of the paint is failing, change plan.Strip it, or over-board the wall with plasterboard fixed through to the studs or masonry and skim that. Over-boarding is very often the faster, more certain route to a flat wall.

Tips

• Over-boarding also gives you the chance to run new cables or add insulation, which makes it a better use of the disruption.
• Test the tape adhesion in the worst-looking spots, not the best. The wall is only as good as its weakest area.
• A wall that has been painted many times over decades has multiple bond lines. Any one of them can be the failure plane.
• If the wall is sound but only slightly uneven, skimming may not be the answer at all — filling and sanding is a much smaller job.

Common mistakes

• Skimming straight onto vinyl silk — Silk paint is designed to be non-absorbent and easy to wipe. There is no suction and no key, so the plaster relies entirely on the bond between the paint and the wall — and that bond was never designed to carry the weight of a plaster coat. It fails in sheets, sometimes weeks later.
• Using PVA to bond plaster to paint — It adds a film between two smooth layers rather than creating a key. Grit-filled bonding primers exist precisely for this situation and are not expensive.
• Sanding old paint in a pre-1980 house — Lead paint sanded dry produces fine lead-bearing dust that settles across the house and is very difficult to remove. It is a genuine health hazard, particularly for children, and it is why testing first matters.

If it doesn't work

Questions people ask