The plumber has been, the roof is repaired, the stain is still there. It is tempting to paint straight away. Painting too early traps water behind the paint and the stain comes back. This guide explains how to confirm the cause has ended, what the sources say about drying and measuring moisture, and what the technical data sheets ask for before sealing and painting.
Updated on 4 November 2026. General information, not a substitute for an assessment by a professional.
The essentials
- First, the cause must be fixed. Cleaning or painting without that does not help: mould and stains come back, says the US environmental agency (EPA).
- There is no number of days that fits every wall. LNEC (Portugal’s national civil engineering laboratory) found that thick stone walls hold liquid water for a very long time.
- Barbot’s Branco Total sheet asks for substrate moisture below 5 % and air humidity below 80 %. It does not say how to measure this at home.
- A handheld moisture meter is for comparing, not for an absolute value (LNEC).
- Stains are sealed only when they are dry. The sealer does not repair the wall.
1. Has the leak really stopped?
A leak can stop dripping and still wet the wall from inside. Before thinking about paint, check:
- Who repaired it should say what they did and where. Ask for it in writing and keep before-and-after photos. If insurance is involved, read home insurance, damp and leaks before repairing or painting, so you do not lose evidence.
- The stain does not grow and no new one appears after rain, or after using the shower or washing machine (everyday observation, not in the data sheets).
- If the water came from another flat, the repair falls to whoever caused it or to the condominium. See leak from upstairs: who pays.
LNEC gives one clue for telling causes apart: if the wall has a high water content in both summer and winter and no pipe is broken, the source may be capillary rise. See rising damp treatment. If the damp is condensation, paint will not fix that either: the three types of damp.
2. Drying the wall
The EPA says wet areas should be dried within 24 to 48 hours to prevent mould. That is a rule for recent floods and leaks, from an American body. We found no equivalent official rule in Portugal.
After that, the material matters:
- The EPA notes that absorbent, porous materials such as ceiling tiles may have to be thrown away if they grow mould.
- Thick stone or brick walls: LNEC measured in old stone walls that the relative humidity inside the wall stayed high long after wetting. The surface can look dry while the inside is not.
- Ventilation helps. If you use a dehumidifier, see when it helps.
- Mould must be dealt with before painting. See mould on walls. The Swiss Federal Office of Public Health (FOPH) says a proper repair removes the cause, removes the mould and dries the damp materials.
3. Measuring moisture
| Method | What LNEC says | Use at home |
|---|---|---|
| Handheld meter (two pins) | Measures electrical resistance. Use it for comparison, not as an absolute value. On renders, only readings near zero (dry) or the top of the scale (very wet) are reliable. Salts change the result | Compare the repaired area with a dry area of the same material |
| Gravimetry (weigh a sample, oven-dry, weigh again) | The most reliable method, but destructive | Only with a technician or laboratory |
| Relative-humidity probes in drilled holes | Give humidity inside the wall | For technical diagnosis |
In practice a DIY meter gives a number, but LNEC warns that its scale is for defined materials (concrete, plaster, wood) and that on renders it is for comparing. When measuring, always compare with a dry wall in the same house, same material. If the repaired area is still clearly above the dry one, wait. This is our reading of what LNEC says, not a rule from the data sheets.
The one concrete limit we found is in the Barbot Branco Total sheet (28/02/2025): to apply and dry, ambient temperature above 5 °C, air humidity below 80 %, substrate moisture below 5 % and substrate temperature 2 to 3 °C above the dew point. The sheet names neither the instrument nor the scale. So we cannot say what reading on your meter corresponds to 5 %.
4. Sealing the stain and painting
With the cause fixed and the wall dry:
- Brush off anything loose. The sheets ask for dry, firm surfaces without dust or grease.
- Apply a stain blocker to dry damp stains. CIN names Stop Manchas (July 2013): 1 to 2 coats, touch-dry in about 4 hours. Aqua Primer (February 2020) can be recoated after 4 to 6 hours, at 20 °C and 60 % humidity. Both ask for air above 10 °C and humidity below 80 %. Below that, they block less well.
- Paint from wall to wall. The technique is in painting ceilings, which also works for walls.
- Watch for a few weeks. If the outline reappears, there is still water.
New render to repair the wall: the CIN Ediplás sheet (revision July 2026) says that on new exterior cement renders you should wait for the cement to cure fully, about 1 month. We did not read an equivalent period for interior plaster or lime.
If the stain has efflorescence (white powder), the problem is different: see salt on walls. If paint peels or blisters, see peeling paint.
What we could not confirm
- How many days to wait after repairing a leak: none of the official Portuguese sources we read gives a period.
- What meter reading means “dry” for household renders: LNEC speaks only of comparison.
- Curing time of interior plaster: we found no figure.
- The 24 to 48 hour period comes from the EPA, for mould after water, not from a Portuguese standard.
- How long until a stain returns: the data sheets give no data.
Sources (read on 4 November 2026)
- EPA, A Brief Guide to Mold, Moisture, and Your Home: drying in 24 to 48 hours, fixing the water source. No date on the page.
- LNEC, Metodologia de diagnóstico de anomalias devidas à humidade em paredes antigas (methodology for diagnosing damp problems in old walls), report 115/2008-NRI, March 2008: sections 3.3 and 3.4 (probes and moisture meter) and 5.2 (gravimetric method).
- Swiss Federal Office of Public Health (FOPH), Bolor em espaços habitáveis (Portuguese edition), February 2021: recommendations to landlords.
- Barbot, Data sheet 1743 Branco Total, 28/02/2025: ambient conditions.
- CIN, Stop Manchas 45-335, July 2013; Aqua Primer 12-830, February 2020; Ediplás 29-830, July 2026.