Does painting over mould work? Why it fails

No, painting over mould does not work. The paint hides the stain for a while, but the mould is still underneath and so is the cause of the damp. After a few weeks or months, the stain comes back.

What happens when you paint over it

Mould lives on the surface and inside the wall. If you paint without cleaning it first, the paint bonds poorly, because it is sticking to a weak, dirty layer.

The dark stain can show through the paint, especially if it is a light colour.
The mould keeps growing underneath and reappears.
The paint can blister, crack or peel, because the damp keeps pushing against it.
The musty smell does not go away.
You lose the money and the work put into the painting, because you will have to redo it.

The cause is still there

Mould is only the sign. The real problem is damp. If the wall keeps getting water, whether from condensation, a leak or the ground, the mould comes back even with fresh paint.

So, before thinking about paint, find out where the water is coming from. If you are in doubt, read our article on how to tell condensation, leaks and rising damp apart.

The right steps

Find the cause and fix it. It might mean ventilating better, repairing a leak, sealing a window or cleaning a gutter.
Clean the mould wearing gloves and a mask, with the window open. See the article on how to clean mould safely.
Let the wall dry completely. This can take days or weeks, depending on the wall and the time of year.
Scrape off loose and blistered paint, and repair cracks and weak plaster.
Apply a primer suited to the surface.
Paint with your chosen paint, in two coats.

Primer is a first coat that helps the paint to bond and evens out the wall. It is not a finishing paint.

Will anti-mould paint solve it?

Some paints contain additives that make it harder for mould to grow on the paint film. They help, but they do not replace cleaning and they do not solve damp. If the wall is wet, even these paints can fail. We have a separate article on this subject.

When the wall never dries

If the wall is still damp after a long time, or if the mould comes back soon after cleaning, the cause has not been dealt with yet. In these cases, it is better to ask for an assessment before repainting. You may need to repair a roof, treat a façade, redo some plaster or improve the ventilation.

In old buildings in Lisbon and Amadora, it is common to find thin or poorly insulated external walls. Near the sea, in Cascais and Oeiras, sea spray makes the damp worse. In the Sintra area, the damper climate makes walls dry more slowly. In all of these cases, it is worth giving the wall time before painting.

Tip: Before painting, put your hand on the wall. If it feels cold, damp or smells musty, do not paint yet. A dry wall holds the paint better and it lasts longer.

Painting over mould saves time now and creates more work later. What works is to deal with the cause, clean, dry and only then paint.