There is no single "damp wall treatment" — there are three, because internal damp has three different causes. Seepage from outside, moisture rising from the ground, and condensation from the room's own air each need their own cure, and the wrong cure fails while looking busy.
First, name the cause
The full separation method is in Dampness or Leakage? — in brief: rain-linked patches with an outside source are seepage; a low tide line with salt is rising damp; corner and cold-surface damp in closed humid rooms is condensation. Two minutes of honest looking beats two coats of hopeful product.
Treatment by cause
Seepage
- Find and fix the entry outside — the tracing guide.
- Let the wall dry through. Weeks, not days; masonry holds water long after the surface pales.
- Repair inside: remove damaged plaster and salt-contaminated material, replaster where needed.
- Repaint last, on a dry wall.
Rising damp
Treatment addresses the wall base — drainage and ground levels outside, damp-course measures in the masonry. What does not work: sealing the room face, which pushes the moisture higher. This one is worth professional assessment before spending; send photographs of the tide line and the outside base via the waterproofing page.
Condensation
The cure is the room, not the wall: ventilation across it, moisture sources managed, air moving behind furniture kept off cold corners. Chemicals have no role — the honest answer nobody profits from except you.
The step everyone rushes
Repainting before the wall has dried. Fresh paint over residual moisture blisters within the season and convinces the owner the "treatment failed" — when only the patience did. A wall is ready when it has stayed visibly dry through a spell of the weather that used to wet it.
Frequently asked questions
Does damp-proof paint fix a damp wall?
It decorates one. If the cause is cured and the wall is dry, good paint keeps it looking right; over an active cause it is a season's disguise (the full distinction).
The plaster feels soft and powdery. Repaint?
No — soft, salt-laden plaster has failed and will shed any paint. It comes off and is redone as part of the repair.
How long does the whole cycle take?
The fix can be quick; the drying is the schedule. Build the repaint date around dryness observed, not days counted.