Mould on a wall can be cleaned in twenty minutes. Helping to keep it from coming back is another matter. If you just scrub it with bleach and paint over it, in two or three months you'll have the same black stain in the same place. This guide explains how to clean it properly, why it reappears and what you need to change to help stop it coming back.
What exactly is the mould you see
The black, greenish or grey stains that appear in corners, on ceilings and behind wardrobes are colonies of fungi. Their spores are in the air of every home. They only germinate when the surface stays damp for days on end, with relative humidity at the wall above 75–80%. In other words, mould isn't the cause of the problem: it's the visible symptom that the wall is cold and damp for long periods.
Before cleaning, it's worth knowing where the damp comes from. In most homes mould is the result of condensation, but it can also come with water ingress. If you're not sure, spend five minutes on the guide how to tell what kind of damp you have or send us a photo for a free diagnosis.
How to clean mould step by step
- Protect yourself. Gloves, an FFP2 mask and an open window. The spores you stir up when scrubbing are the most irritating.
- Don't brush it dry. You'll spread spores around the whole room. Wet the area with the product first.
- Apply the cleaner. On painted walls, diluted bleach (one part bleach to four parts water) or 3% hydrogen peroxide work well. Spray, leave for 10–15 minutes and wipe off with a sponge. Never mix bleach with ammonia or vinegar: it gives off chlorine gas.
- Rinse and dry. Wipe with a cloth and clean water and dry the area with another cloth. Leave the window open until the wall is dry to the touch.
- Check the surface. If the mould has got into the plaster or a plasterboard panel (the surface crumbles or smells when you touch it), cleaning isn't enough: scrape back to sound material and repair.
| Cleaner | Dilution | Good for | Watch out for |
|---|---|---|---|
| Bleach | 1:4 in water | Painted walls, tiles, grout | Discolours, don't mix with other products |
| 3% hydrogen peroxide | Undiluted | Plaster, wood, fabric | Slower, repeat if needed |
| White vinegar | Undiluted | Small surfaces, silicone | Not very effective on porous plaster, smell |
| Commercial fungicide | As on the label | Large or recurring areas | Ventilate well, not a permanent treatment |
Bleach kills mould on the surface, but it doesn't penetrate porous materials. If the wall is still damp inside, the hyphae left in the plaster grow back.
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Why mould comes back
Cleaning removes the colony, not the conditions that created it. Three reasons explain almost every relapse:
- The wall is still cold. A north-facing corner, an uninsulated column or a slab edge are several degrees colder than the rest of the wall. The damp air in the room condenses there every night.
- The existing paint is plastic. Conventional acrylic paints form a closed film. Water condenses on it and stays there; it's the ideal surface for the fungus to grow back.
- You used paint with biocide. It works while the fungicide is active, between six months and two years depending on the product and exposure. Once it runs out, the mould reappears because the wall hasn't changed.
When mould is condensation
If you recognise several of these signs, the cause is condensation and the solution lies with the wall and the air, not building work:
- It appears in upper corners, where the wall meets the ceiling or behind furniture against outside walls.
- It shows up in autumn and winter and improves in summer.
- There are drops on the windows in the morning (if that's your case and it appears in the corner of the bedroom ceiling, there's a specific guide: mould in the bedroom).
- The affected area is dry to the touch during the day but has scattered black spots, not a stain with a clear tide mark.
- There's no white powder or blistered paint low down on the wall (that points to salt bloom).
If instead you see a stain with a clear edge that grows after rain or gathers around a window, a downpipe or a terrace, it's water ingress: water is getting in from outside and that has to be stopped first. It's covered in detail on the condensation page and the water ingress page.
Painting after removing the mould
The order matters. If you paint a wall that's still damp, the paint won't bond and mould will grow underneath.
- Wait until it's dry. At least 24–48 hours after cleaning, with the room ventilated. In winter and with high humidity, wait longer.
- Repair whatever needs repairing. Scrape off loose paint, fill damaged areas, sand and remove the dust.
- Choose a breathable, anti-condensation paint. MATE5000 is a breathable, anti-condensation interior paint with a deep matt finish. According to its technical data sheet, it helps reduce the surface conditions that encourage condensation. If the area had mould, apply Sellador Total L44 first: it regulates absorption and improves adhesion of the finish. Both come together in the Condensation Kit for €26.69, 6% less than buying them separately. If you'd prefer a finish that also works in sheltered outdoor areas, have a look at Pared Seca.
- Two coats. A 4 L tin covers around 10–14 m² in two coats (5–7 m²/L per coat), enough for a bathroom or a large wall, and costs €19.02. Allow the drying time between coats given in the technical data sheet.
If you're only painting one area, bear in mind the shade may differ from the rest of the wall: the neatest option is to paint the whole wall from corner to corner.
No paint, however good, removes moisture from the air. What MATE5000 does is change how the wall surface behaves: it's breathable and anti-condensation. To make the result last, combine it with daily ventilation: ten minutes with the window open in the morning, extractors in the bathroom and kitchen, and furniture kept a few centimetres away from outside walls.
When you need a professional
There are cases where cleaning and painting isn't enough and you should have someone look at the home:
- The mould covers more than a square metre and comes back in under two months after being cleaned properly.
- Plasterboard panels are soft, warped or crumbling.
- The stain grows with rain: water is getting in and paint won't fix that.
- There's damp in the floor or low down on the walls of a ground floor.
- Someone at home has asthma or allergies and the symptoms get worse in that room.
In those cases, before buying anything, send us photos. We'll tell you whether paint will sort it, whether you need a sealer or a primer first, or whether it's time to call a builder.
Guía escrita y revisada por el Equipo Humix Paint con los datos de las fichas técnicas de cada producto. La pintura actúa en la superficie: si el agua sube del suelo o entra de fuera, primero se trata la causa.
