Why does my paint rub off as powder when I wipe the wall?

A paint film is pigment held together and stuck to the wall by binder. The pigment gives colour and hiding; the binder gives everything else. Reduce the binder — by buying a paint that has little, by diluting it, or by letting sunlight break it down — and the outer pigment particles are left with nothing holding them, so they come away on a hand, a cloth or a sleeve.

What you'll need

• A stiff brush and a vacuum
• A stabilising solution or masonry stabiliser
• A durable emulsion or the correct exterior finish
• Buckets, cloths and a mild detergent

Step by step

1. Do the wet finger test.Draw a damp finger across the wall. If colour transfers, the film is chalking. If nothing comes off but the wall looks dusty, it is surface dust rather than a failing film. If a wet finger makes the surface milky and it comes away as slurry, that is distemper, which is a different problem again.
2. Work out which of the three causes you have.Indoors, a wall that has always rubbed off since it was painted is almost certainly contract or trade matt emulsion left as the finish, or a mist coat thinned too far and then left. Outdoors, chalking on masonry paint after several years is normal UV ageing of the binder. In either case, adding a coat directly on top is the wrong move.
3. Remove everything loose before anything else.Brush the surface down hard with a stiff brush, vacuum it, and then wash it with water and a little detergent, rinsing until the cloth stops coming away coloured. On exterior masonry this may take a pressure wash at a modest setting followed by full drying.
4. Bind what is left with a stabilising solution.This is the step that makes the difference. A stabilising solution or masonry stabiliser soaks into the remaining powdery layer and binds it to the substrate. Apply it to the whole surface, not just the worst areas, and let it dry as the tin specifies. Without it, your new paint is bonded to loose dust.
5. Test before you commit the whole wall.Once the stabiliser is dry, apply the finish to a square metre, let it dry a day, then press strong tape onto it and pull it off. If paint comes away, the surface needs another stabiliser coat or the old coating needs to come off entirely.
6. Repaint with a paint that has enough binder for the job.Indoors, a durable or scrubbable matt or a soft sheen — not another contract matt. Outdoors, a good masonry paint over the stabilised surface, chosen for the wall's breathability requirement.
7. Expect a whole-wall job.Chalking does not have a boundary. Treat the entire wall or the entire elevation, corner to corner, because a stabilised patch next to an untreated one gives you a suction difference that will show through the new paint.

Tips

• New-build walls that rub off are the single most common version of this. The builder used contract matt as a finish. Wash, stabilise if it is heavily chalking, and put two coats of a durable emulsion over it.
• Stabilising solution is not the same as PVA and PVA is not a substitute — PVA re-softens with moisture and can cause the new film to peel.
• Outdoors, some chalking is by design in traditional mineral and limewash finishes. That is the coating weathering as intended, not failing.

Common mistakes

• Painting straight over a chalking wall — The new film bonds to loose powder rather than to the wall. It comes off later in sheets, taking the powder with it, and you have made the eventual repair bigger.
• Using PVA instead of a stabilising solution — PVA softens when it gets damp and provides a slip plane rather than a bond. Use the product designed for the job.
• Stabilising only the worst patches — You create areas of very different suction across one wall, which shows as flashing and patchiness under the topcoat.

If it doesn't work

Questions people ask