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Why damp has to be sorted before the paint goes on

You have stripped the wallpaper, filled the cracks, sanded the filler back and stood there with a roller in your hand — and then you spot it. A brown tide mark creeping up from the skirting. A powdery white bloom across the plaster. Or that unmistakable black speckling in the corner of the room. It is tempting to slap a coat of stain block over it and pretend you never saw it, but painting over a damp wall is a false economy. Trapped moisture will push the paint off from behind, blistering the finish, staining it and flaking it onto the floor within a season or two. Do the groundwork properly and your decoration will last for years.

Track down the source of the moisture

Paint is a surface fix. Until you know why the wall is wet, anything you apply is just decoration over a problem. In a typical UK home the culprit is usually one of a handful of things:

• Rising damp — confined to ground-floor walls, usually the bottom metre or so, often with a horizontal tide mark and salty, crumbling plaster.
• Penetrating damp — higher up the wall, in patches. Look for failed pointing, cracked render, a leaking gutter or downpipe, or a chimney that needs attention.
• Bridged damp-proof course — soil, paving or a raised patio sitting above the DPC, giving moisture a path straight into the wall.
• Condensation — the most common of all. Black spotty mould in cold corners, around windows and in bathrooms and kitchens, worst in winter.
• Leaks — a slow drip from a pipe or a shower tray can travel sideways a surprising distance before it shows itself.

Two quick tests help. A cheap moisture meter gives you a reading at various heights up the wall. Or tape a square of polythene to the plaster for a few days: condensation on the room side points to a ventilation problem, damp on the wall side means water is coming through the fabric.

Let the wall dry out — properly

This is the part most people rush, and it is the part that matters most. Damp plaster and masonry cannot be painted, sealed or skimmed until they are genuinely dry. As a rough rule of thumb, allow about a month per inch of plaster thickness, and longer for solid brick walls that have been soaked for years. A moisture meter reading of around 5% or lower is a sensible target before any paint goes anywhere near it.

To speed things along without causing damage:

• Open windows daily and keep the room ventilated rather than sealed up.
• Use gentle background heating — never blast a fan heater directly at plaster, as rapid drying causes cracking and draws salts to the surface.
• Run a dehumidifier, emptying it regularly.
• Lift the skirting if it is sodden; it traps moisture against the wall and will need replacing anyway.
• Fix the fault first. Drying out a wall while a gutter still overflows is a waste of time.

Treat the surface: mould, salts and friable plaster

Once the wall is dry, deal with what the damp has left behind. Wash down any mould with a proprietary fungicidal wash, following the instructions and wearing gloves and eye protection. Keep the window open — these products are not pleasant to breathe.

White, fluffy salts are common after rising damp. Brush them off dry, then seal the surface with an alkali-resisting primer, which stops the salts staining your topcoats. If the plaster is chalky, dusty or crumbly, apply a stabilising solution to bind it together — painting straight onto friable plaster is a guarantee of flaking. Where the plaster sounds hollow, has blown, or crumbles when you touch it, it is best to hack it back and re-plaster, ideally with a renovating plaster designed to tolerate residual moisture.

Choose the right primer and paint system

Different problems need different products, so match the treatment to the cause:

• Condensation only: improve ventilation, insulate the cold surface if you can, then decorate normally with a mould-resistant emulsion.
• Rising or penetrating damp that has been repaired: if the wall may stay slightly damp, use a surface damp-proof membrane — a two-part epoxy or liquid-applied coating — then skim, line or fill over it before painting.
• Exterior masonry: once dry, prime bare or chalky areas and finish with a good-quality breathable masonry paint in two coats.
• Old solid walls: these need to breathe. A breathable finish such as a mineral or clay paint lets moisture escape rather than trapping it, which a modern vinyl emulsion will not.

Always read the drying times on the tin. Rushing between coats, especially in a cold room, is a classic way to ruin an otherwise decent job.

When to call in a specialist

If the diagnosis points to rising damp, an injected damp-proof course is usually the answer — a job for a qualified damp specialist, normally backed by a guarantee. The same goes for structural cracks, failed drainage or anything that requires digging into the fabric of the building. Sorting those out now, before the first roller stroke, is far cheaper than redecorating the same wall again next year.