What it is
Masonry contains soluble mineral salts. Cement, render, brick and mortar all have them.
When moisture moves through a wall, it dissolves those salts and carries them to the surface. The water evaporates. The salts don't — they're left behind as a white crystalline deposit.
That's efflorescence. The salt itself is harmless. What it tells you is not: there is moisture moving through that wall.
Where the moisture comes from
- A failed or bridged damp-proof course
- Rainwater penetration through cracked render or failed pointing
- A plumbing leak, often a slow one behind a wall
- Ground moisture, particularly where a terrace or path is above internal floor level
- Condensation, especially in bathrooms and around window reveals
- On coastal property, high ambient humidity combined with salt-laden air
Why paint won't fix it
Three reasons, and they're worth understanding because they explain every failed attempt:
- The salts keep coming. As long as moisture is moving through the wall, more salt arrives at the surface. It crystallises under the paint film and pushes it off.
- Nothing bonds to it. Efflorescence is loose powder. A coating applied over it is bonded to the powder, not the wall.
- It's a symptom. Covering it up doesn't stop what's causing it, and moisture in a wall causes more than cosmetic problems over time.
The right sequence
Find the source. This is the whole job. Until the moisture is stopped, everything else is temporary.
Let the wall dry. Properly. Weeks rather than days, depending on how wet it is. Painting a damp wall guarantees failure.
Remove the deposits dry. Stiff brush, dry. Do not wash it off — washing dissolves the salts and carries them straight back into the wall, where they'll reappear as soon as it dries again. This single mistake is behind an enormous number of repeat failures.
Treat the surface. An efflorescence-resisting primer or alkali-resisting sealer, appropriate to whether the wall needs to breathe.
Then paint — with a breathable coating if there's any residual moisture in the structure.
When the source can't be fully stopped
Sometimes it can't. In that case the honest approach is a breathable system that lets the wall release moisture, on the understanding that some marking may recur. That's a managed situation rather than a fix, and it's better to be told that at the quote than to discover it a year later.
Beware the quick fix
Damp-seal and waterproofing paints have their place on a dry wall with a mild historic problem. On an active moisture source they generally make things worse — they trap the water, which then comes out somewhere else, often through the plaster on the other side of the wall.
Getting it looked at
If you've painted the same patch more than once, stop and get it looked at properly. It costs nothing and it's a different conversation from a straightforward repaint.
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