Mould on walls: why painting over it never works

Mould is not a stain. It's a living organism with roots growing into your wall. Paint hides the colour; it doesn't kill anything. That's why it comes back, usually within weeks, and usually worse.

This is one of the most common jobs we're called to fix in Leicester, and almost always the same story: someone painted over the black patch in the bedroom corner, it looked fine for a month, and now it's back and spreading past where it started.

Why paint alone never works

Mould is a fungus. What you see on the wall is the fruiting body; what you don't see is a network of hyphae growing into the porous surface of the plaster or the paint film. Rolling emulsion over it does three things, none of them good:

• It feeds it. Emulsion contains organic binders and cellulose thickeners. Mould will happily eat them.
• It traps moisture. The condensation causing the mould is still arriving. Now it has a paint film holding it against the wall.
• It doesn't kill anything. The growth continues underneath and pushes back through, often within four to eight weeks.

You also can't paint the stain out. Mould staining bleeds through emulsion no matter how many coats you apply, because the pigment is water-soluble and travels up through each wet coat. That's a job for a stain blocker, and only after the mould itself is dead.

First, work out what you're actually dealing with

This matters more than anything else in this article, because it decides whether you need a decorator at all.

The vast majority of mould in Leicester homes is condensation, not rising damp. It shows up on cold surfaces where warm, moist air meets them: north-facing external walls, corners where two external walls meet, and behind furniture pushed tight against an outside wall where air can't circulate.

How mould should actually be treated

Done properly, it's a sequence and each step matters:

1. Fix the source. If nothing changes about the moisture, everything below is temporary. More on this in a moment.
2. Protect yourself. Gloves, and a mask if the area is significant. Disturbing mould releases spores.
3. Apply a fungicidal wash. A proper biocidal solution, not bleach. It needs dwell time, usually 15 to 30 minutes depending on product, to actually kill the growth rather than just wet it.
4. Remove the residue. Wipe away dead growth. Don't scrub aggressively while dry; that just launches spores into the air.
5. Let it dry completely. Genuinely dry, not "feels alright". This can take days on a cold external wall.
6. Seal the staining. A stain block prevents the residual discolouration bleeding through your finish. Emulsion will not hold it back.
7. Finish with an appropriate system. A fungicidal or anti-mould emulsion in the affected area, or a quality durable matt where conditions have been improved.

Skip any step and you get a callback. Most commonly, people skip the dwell time and the drying, because both mean waiting.

Why bleach isn't the answer

Bleach is the internet's favourite mould advice and it's the wrong tool for plaster. Household bleach is mostly water. On a porous wall, the water carries spores deeper into the surface while the chlorine, which does the killing, stays on top and evaporates. You get a wall that looks clean and is still colonised below the surface.

Bleach works reasonably on non-porous surfaces: tiles, glass, silicone, glazed surfaces. On plaster and painted walls, use a dedicated fungicidal wash designed to penetrate. It costs a few pounds and it actually does the job.

Anti-mould paint, honestly

Anti-mould and fungicidal emulsions contain biocides that make the paint film hostile to regrowth. They genuinely help and we use them where they're warranted. But be clear about what they are: they manage a symptom.

If warm moist air keeps condensing on a cold wall, mould will eventually reappear, often at the edges of the treated area, or behind the film. Anti-mould paint buys you years instead of months. It doesn't buy you a permanent fix if nothing else changes. Anyone selling it as a cure is overselling.

The bit that actually stops it coming back

Condensation mould has three ingredients: moisture in the air, a cold surface, and still air. Remove any one and it struggles. Practical changes that genuinely work in Leicester homes:

• Use the extractor, and use it longer. Run bathroom and kitchen fans during and for 20 minutes after. Most people switch them off with the light.
• Close the door, open the window. When showering or cooking, contain the moisture in that room and vent it out, rather than letting it migrate to a cold bedroom wall.
• Move furniture off external walls. Even 5cm of air gap behind a wardrobe makes a measurable difference. Mould behind furniture is one of the most common patterns we see.
• Dry washing outside or vented. A load of wet washing on a radiator puts litres of water into your air.
• Low, steady heat beats bursts. Cold walls are the problem. A house heated briefly to 22°C and then left cold cycles through condensation daily.
• Trickle vents open. They exist for a reason and most people tape them shut.

Where this fits in a decorating job

Mould treatment isn't a service we sell for its own sake. It's a preparation step. If there's mould on your bedroom wall, treating it properly is simply part of decorating that room correctly. It's the same logic as filling a crack: nobody wants "crack filling" as a product, they want a wall that looks right in three years.

When you use our instant quote, mould treatment appears as a clearly priced line where it's needed, with an explanation of what it is. You can see exactly what it costs, and decide. We'd rather show you the number than hide it in a lump sum, and we'd rather tell you it's a damp problem than take the job.

Frequently asked questions