What is the white powder coming through my painted wall?

The powder is not the problem — it is the receipt. Salt cannot move through a wall without water to carry it, so a bloom of white crystals on a wall is direct evidence that moisture is travelling through that wall and evaporating at the surface. Everything useful follows from finding the water.

What you'll need

• a stiff dry brush
• a vacuum with a filter
• a torch
• a moisture meter (optional)
• a salt-resistant or breathable coating, if appropriate (optional)

Step by step

1. Confirm it is efflorescence and not mould or bloom.Efflorescence is a dry, crystalline, usually white powder that brushes off. Mould is soft, often coloured, smells, and returns as a growth. Lime bloom on new render or concrete is similar but tends to be more of a uniform whitish film.
2. Look at where it is on the wall.A band low down on a solid wall points at rising damp or a bridged damp course. A patch high up or around a chimney breast points at penetrating water or salts from soot. Around a window suggests a leak at the reveal.
3. Look for the water source, in order of likelihood.A leaking pipe or waste, a failed seal or flashing outside, a blocked gutter throwing water at the wall, ground level raised above the damp course, or condensation in a cold spot.
4. Check whether the wall is new.New masonry, new render and new concrete all contain salts and a great deal of construction moisture. Efflorescence in the first year of a new wall is common and usually stops once the wall has dried out.
5. Fix the moisture before touching the decoration.Nothing applied to the surface will stop salts arriving. This is the whole job, and everything else is cosmetic.
6. Let the wall dry fully.Drying a wall takes far longer than most people expect — weeks or months depending on thickness and conditions. Ventilation and gentle background heat help.
7. Brush the salts off dry, and vacuum them up.Do not wash them. Water redissolves the salts and carries them back into the wall, where they come out again later. Brush dry, vacuum, repeat as they reappear during drying.
8. Repeat until the salts stop coming back.A drying wall will keep producing them for a while. Waiting until the production stops is what stops the redecoration failing.
9. Redecorate with a coating suited to the situation.On a solid wall that needs to breathe, a breathable finish is important — a sealing coating traps moisture and pushes the problem elsewhere or blows the plaster. Where the plaster is heavily salt-contaminated it may need replacing with a salt-resistant system.

Tips

• Photograph the affected area and date it. Whether the patch is growing, shrinking or seasonal is the most useful diagnostic information you can gather.
• Salt-contaminated plaster is hygroscopic — it draws moisture out of the air — so it can look damp even after the original leak is fixed. That plaster often has to come off.
• Efflorescence on the outside of a brick wall is common and usually harmless; on the inside it means water is reaching the inner face.
• If the powder tastes salty, is greasy, or the patch has a distinct tide line, that is more consistent with rising damp and worth a proper investigation.

Common mistakes

• Washing the salts off with water — It dissolves them and drives them back into the wall, where they crystallise again and reappear — usually worse, because the water added to the wall gives them a fresh ride to the surface. Brush dry.
• Sealing the wall with a waterproof coating or oil-based paint — The moisture is still arriving; blocking its exit at the surface simply moves the evaporation point deeper into the wall, where the crystallising salts push the plaster off. This is a common cause of blown plaster behind a 'damp seal'.
• Redecorating before the wall has stopped producing salts — The salts come through the new paint, so the entire job is wasted. It is a waiting game, and there is no shortcut past the drying.

If it doesn't work

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