Painting a damp wall has one rule with no exceptions: never paint over active damp. If the wall is wet inside, the paint won't grip, it blisters within weeks and, on top of that, it hides the problem. This guide is about how to prepare the surface so the paint lasts, what to scrape off, how long to wait and what goes on before the paint depending on the type of damp.
Before anything else: check whether the damp is active
A wall may have old stains from damp that has already been resolved, or it may be wet right now. They're treated differently. Two ways to find out:
- Moisture meter. A contact meter costs 20–30 euros. Measure the damaged area and a sound wall in the same home. If the difference is large, the damp is active.
- The plastic sheet test. Tape a 50 × 50 cm piece of cling film or plastic over the area, sealing all four edges well, and wait 24 hours. Droplets on the inner face: the water is coming from the wall (rising damp or water ingress). Droplets on the outer face: it's condensation from the air in the room.
If you don't know what kind of damp you have, the guide how to tell what kind of damp you have in 5 minutes sorts it out, or send photos and we'll look at it in the free diagnosis.
What goes on before the paint, by type of damp
| Type | Treatment first | Coverage | Afterwards |
|---|---|---|---|
| Condensation | Clean the mould, let it dry; Sellador Total L44 if there was fungi | 9–10 m²/L | MATE5000, two coats |
| Salts / rising damp | Scrape dry, repair, NeutralySalt Primer | 1 L ≈ 25–30 m² | MATE5000, two coats |
| Water ingress through a wall | Find the entry point, repair, Sellador Total L44 | 9–10 m²/L | MATE5000, two coats |
| Water ingress through the roof | Cool Roof Therm on the flat roof | 1–1,5 kg/m² in total | MATE5000 inside |
If the water ingress comes from a structural crack, a broken downpipe or worn-out roof waterproofing, no brush-applied product will fix it. Have the cause repaired by a professional and then carry on with this guide.
MATE5000 AntiHumedad
Breathable, anti-condensation interior paint with a deep matt finish.4 L and 15 L · 12 colours · 5–7 m²/L
Materials
- Filling knife, scraper and stiff-bristle brush
- Medium-grit sandpaper (80–120) and a sanding block
- Vacuum cleaner or brush to remove the dust
- Repair filler or mortar, depending on the size of the damage
- Masking tape, plastic sheeting for the floor
- Short- or medium-pile roller, brush for cutting in, paint tray
- Gloves and a mask if there's mould or dust
- The products that apply: NeutralySalt Primer, Sellador Total L44, MATE5000
Step 1: clean
If there's mould, remove it with bleach diluted 1:4 or hydrogen peroxide, leave it to work and wipe off with a sponge. The details are in how to remove mould from a wall. If there are salts, brush them off dry and vacuum: never with water, because you dissolve the salt and send it back into the wall. If there's grease or nicotine, clean with a neutral detergent and rinse.
Step 2: scrape off what's no good
This is the part most people skip and the one that most determines the result. You need to remove:
- Flaking or blistered paint. Scrape until the edge of the old paint is firm. Pull at it with the filling knife; if it lifts, keep going.
- Hollow render. Tap with your knuckles. Where it sounds hollow, the render has come away from the wall: it has to be removed and redone.
- Soft or crumbling plaster. If it comes apart when you run a fingernail over it, off it comes.
- Warped plasterboard. A board that has swollen won't recover. It's cut out and replaced.
If the old paint is a satin emulsion, sand the whole surface to take off the sheen so the new paint has something to grip.
Step 3: repair and leave to cure
Fill the holes. For small chips, ready-mixed filler. For areas where you've removed render, repair mortar, preferably lime-based if the wall has had salts. Apply in thin layers if the hole is deep. When it's hard, sand it flush and remove all the dust.
Approximate curing times before priming or painting:
- Fine filler: 24 hours
- 1 cm of mortar: 5–7 days
- More than 2 cm of mortar: two weeks or more
In the Canary Islands and the north of mainland Spain, where ambient humidity is high, these times are longer. The wall has to be dry to the touch, with no difference in colour between the repair and the rest.
Step 4: prime if needed
For condensation, over well-bonded existing paint no primer is needed; if there was mould, apply Sellador Total L44 diluted 1:1 and wait 24 h before painting. For salts, apply NeutralySalt Primer over the whole affected area plus a margin of 30–40 cm above it. For water ingress through a wall, once the entry point has been repaired and the wall is dry, Sellador Total L44 as a primer. Respect the drying time on each product's technical data sheet before painting over it.
Step 5: paint
- Stir the tin well. The product may thicken in the container without losing its properties: shake it before use.
- Cut in the corners, ceiling and skirting board with a brush.
- First coat with a roller in criss-cross passes, without overloading it. On a repaired or very absorbent surface, this coat soaks up part of the paint; that's normal.
- Leave it to dry for the time between coats stated on the technical data sheet. If the air is very humid, allow longer.
- Second coat, more even. This is where the colour evens out.
A 4 L tin of MATE5000 covers around 10–14 m² with two coats (5–7 m²/L per coat) and costs 19,02 €. Allow 10–15 % more if the surface is rough or you've done a lot of repairs. The calculator works it out for you room by room.
How many coats
Two at least: the technical data sheet says 2 or 3 coats. A third especially if you're going from a dark colour to a light one, if the repair still shows clearly or if the first coat was absorbed unevenly. If you're changing colour, remember that on-screen colours may differ from the real thing.
Mistakes that ruin the job
- Painting while the wall is damp inside to "cover up" the stain.
- Using emulsion paint or paint with biocide over condensation or salts.
- Washing off salts with water.
- Painting over loose paint without scraping.
- Painting late in the afternoon in fog or high humidity: the coat doesn't dry properly and leaves marks.
- Pushing the wardrobe back against the wall the next day. Wait for the paint to cure and leave a 5 cm gap.
When to call a professional
If scraping reveals cracks that run right through the wall, if the hollow render covers more than a square metre, if the plasterboard has disintegrated or if the damp is still active after the preparatory treatments, stop. Paint is the last step, not a substitute for a repair. You'll find the set of products for each type in the anti-damp kit, and if you'd like us to confirm what your wall needs before you buy, send photos.
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.
