Mold on walls is rarely solved with a single product. Most failures come from mistakes in method, not from a lack of bleach. These are the ones that come up most often and how to correct them.
Mistake 1: cleaning without stopping the moisture
Spraying fungicide on a wall that is still receiving water is throwing the product away. Mold needs high relative humidity and a surface with nutrients (acrylic paint, wallpaper, accumulated dust). If the wall is cold or damp to the touch, you first need to ventilate and raise the temperature. According to documentation from anti-mold paint manufacturers, most products require application on a dry surface and at above 10 °C; doing it on a cold wall reduces their effectiveness.
Mistake 2: using pure bleach on paint or plaster
Household bleach (sodium hypochlorite) attacks pigments and can leave yellowish stains on emulsion paint. It also does not penetrate porous plaster: it only lightens the surface layer and the mold reappears within weeks. For bare plaster, a specific product with quaternary ammonium or hydrogen peroxide is better. If in doubt, test on a 20 x 20 cm area and wait 24 hours before spreading it.
Mistake 3: scrubbing dry or with an abrasive scouring pad
Scrubbing dry releases spores into the air, which settle on other walls and on textiles. And the green scouring pad scratches the paint, leaving micro-grooves where mold takes hold better next time. The right way: dampen first, use a microfiber cloth or a soft brush, and work from the bottom up so you don’t drag spores down by gravity. With wallpaper, scrubbing is almost never advisable: if there is widespread mold, the real solution is to remove the paper.
Mistake 4: not drying or ventilating afterwards
A specific case that is seen a lot: a room with a wardrobe pushed against the exterior wall. The stain is cleaned, but the furniture keeps blocking ventilation and the wall doesn’t dry. The mold comes back in the same spot. Leave at least 5 cm of separation between furniture and cold walls, and ventilate for 10-15 minutes a day in winter. In bathrooms without a window, an extractor fan with a timer is more useful than opening the door.
Mistake 5: painting over without treating
Covering a stain with ordinary paint is the most expensive mistake: the paint doesn’t adhere to mold and peels off within a few months. The correct sequence is to clean, let it dry completely (24-48 h depending on thickness), apply a bonding primer and then anti-mold paint or mineral paint. Anti-mold paint does not kill existing mold; it prevents it from appearing. This doesn’t work on walls with active leaks: there you first have to repair the leak.
Mistake 6: mixing products without any criteria
Bleach with ammonia generates chloramines, which are toxic. Bleach with vinegar also produces irritating vapors. White vinegar works well on non-porous surfaces (tile, glass), but on plaster it barely penetrates. And baking soda only cleans, it doesn’t disinfect. Choose one method and don’t combine it with another in the same pass.
Mistake 7: ignoring the structural cause
If mold appears on the lower part of an exterior wall, there is usually a thermal bridge or rising damp. No cleaner will solve the problem there. According to studies from the building sector, rising damp and thermal bridges are behind a good part of recurring mold in homes. In those cases, the solution is insulation or ventilation, not more product.
What does work, in order
1. Protection and preparation
Gloves, FFP2 mask and goggles. Ventilate the room. Remove furniture and textiles from the area.
2. Cleaning in stages
Vacuum with a wet vacuum or dampen before touching. Apply the product, let it act for the indicated time (no less) and wipe off with a clean cloth. Repeat if dark residue remains.
3. Real drying
Ventilate and, if possible, use a dehumidifier. The wall must be dry before painting or sealing.
4. Prevention
Keep relative humidity below 60 %, check ventilation grilles and avoid drying clothes indoors without ventilation.
Note: chlorine-based products can damage metals and some paints; and black mold on large surfaces or after chronic leaks deserves a professional inspection. No guide replaces an inspection if the problem comes back more than twice in the same spot.