Can I paint over a damp stain?

What happens when you paint over it

If water keeps reaching the wall or ceiling, the stain returns, often within weeks or months. The new paint can blister, peel or grow mould, and the old mark can show through. Worse, hiding the sign means losing the clue to where the water comes from.

Likely reasons the stain comes back

• Water keeps getting in through the roof, flashing, façade or windows.
• A pipe has a slow leak.
• The wall had not dried and was painted damp.
• There is condensation from poor ventilation, and paint does not change that.

Most people only notice the problem when the stain appears, and the natural urge is to cover it straight away so the room looks presentable again, especially before guests arrive or before putting the home on the market. It is understandable, but the water that caused it keeps working behind the paint, and a second paint job plus the repair you eventually need usually costs more than fixing the cause the first time. If you are a tenant, remember too that repainting over an unresolved stain can make it harder to show your landlord (senhorio) when the problem started.

How to know when you can paint

Before painting, confirm three things:

1. The cause has been identified. You know where the water came from.
2. The cause has been repaired. The leak, the entry of water or the lack of ventilation is sorted.
3. The wall has dried. The surface feels dry and the stain has not grown since the repair.

If any of the three is missing, painting just postpones the problem.

What to do now: the right order

1. Find where the water comes from. Photograph the stain, see when it grows, and check the roof, gutters, façade and pipes.
2. Repair the cause. Fix the leak, the water entry or the ventilation, and let the wall dry. This can take weeks in a damp Portuguese winter, so be patient.
3. Only then paint. With the surface dry and clean, you can apply a suitable paint.

If there is mould or loose paint, remove the loose layer and clean it first, in safe conditions and with the room ventilated.

When to call a professional

Call someone before painting if the stain is growing, if the wall is wet, if the paint is blistering, or if the stain came back after an earlier paint job. Depending on the cause, look for a plumber (canalizador), a builder (pedreiro), a roofer (telhador) or a painter (pintor) for the final finish. Always ask for a quote that says what the cause is and what will be repaired, not only what will be painted.