Should I paint damp seal over a damp wall inside?

The shelves are full of products promising to solve damp with a brush. They fall into two groups: stain blockers, which stop an old mark bleeding through new emulsion, and damp-proofing or tanking paints, which are intended as part of a designed waterproofing system. Neither removes water from a wall, and using either on a wall that is still getting wet buys a few months and costs you the plaster.

Step by step

1. Establish whether the wall is actually dry.Not dry to the touch on a warm day — dry after the cause has been removed and a heating season has passed. Anything else and you are sealing a live problem.
2. Identify which product you have been sold.A stain block or shellac-based blocker is a decorating product for old marks. A damp seal, tanking slurry or waterproof coating is a barrier product intended to be used under a specified plaster system, not under emulsion.
3. Use a stain blocker for its actual purpose.An old water stain, a nicotine mark or a rust bleed on a wall that is now dry will come through emulsion repeatedly. One coat of an appropriate blocker stops it permanently and that is a legitimate, useful job.
4. Work out what happens to the moisture if you seal a wet wall.It does not stop arriving. It accumulates behind the coating, saturates the plaster, and eventually blows the coating and the plaster off together, over a larger area than the original patch.
5. Think about salts.If the wall is hygroscopically salted, sealing over it is the worst option available. Salt crystals grow behind the film with enormous pressure and lift it off in sheets.
6. Fix the cause instead.Whatever the cause is — gutter, ground level, render, ventilation, a cold surface — it is cheaper to fix than the wall will be to repair twice.
7. Choose a breathable finish for a wall with any history of damp.A mineral, clay or breathable emulsion lets residual moisture leave through the paint film. Vinyl silk and one-coat durable finishes do the opposite and blister.
8. Accept bare plaster for a while if you need to.A mist coat on a repaired wall, left for months, tells you honestly whether the problem is gone. It is the cheapest diagnostic tool in decorating.

Tips

• If a wall keeps needing repainting in the same spot, that is data. Stop repainting and find out why.
• Anti-condensation paint is a different product with a different purpose — a thin insulating layer to raise surface temperature — and it is not a damp seal.
• Tanking slurries do work, in the situation they are designed for, which is a designed system in a cellar or a basement with a drainage strategy behind it.

Common mistakes

• Using a damp seal to get a room ready for a viewing or a tenancy — It hides the evidence for a few weeks and then fails worse, and in a sale or a letting it is the sort of concealment that has consequences beyond the decorating.
• Sealing one wall of a room — In a solid-walled house the moisture simply evaporates somewhere else instead, so the problem moves to the adjacent wall or above the sealed area and looks like a new fault.

Questions people ask