Painted recently and the paint is already coming off in sheets? Blisters, a yellow stain showing through every coat, or powder rubbing off on your hand? Often the paint is not to blame: what failed was underneath it. This guide helps you link each defect to its most likely cause and sets out what the technical data sheets say must be done before you paint again.
Updated on 3 November 2026. General information; it does not replace an assessment by a qualified professional.
The essentials
- Every data sheet we read starts with the same rule: the wall must be dry, sound and free of grease and dust. When any of the three fails, the paint fails.
- Paint that is not firmly attached must all come off before repainting. Painting over a loose layer only adds weight to it.
- Dry stains from water, nicotine or smoke bleed through ordinary paint. They need a stain-blocking primer.
- Mould is never painted over: remove the cause of the damp and treat the wall first. Salt deposits (salitre) are not painted over either: see their own guide.
- Painting in cold or very humid air also ruins the result. CIN, for example, asks for air above 5 °C and relative humidity below 80 % for its Vinylmatt interior paint.
First, look carefully
Before buying paint, note where the defect is: by a window, at the skirting, on a ceiling under a bathroom. The location says a lot about the cause.
| What you see | Likely cause | What the data sheets ask for |
|---|---|---|
| Paint comes off in flakes or sheets | Old layer poorly bonded; dust, grease or damp on the wall | Brush off all loose paint; repair and treat as a new surface |
| Blisters, mostly near the floor, with white powder inside | Salt crystallising behind the paint | Deal with the damp and the salt first |
| Blisters on a bathroom ceiling or wall | Steam and condensation, or a leak | Find the source of the water before painting |
| The wall leaves powder on your hand | Friable surface or old paint that has chalked | Brush and apply a binding primer |
| Yellow or brown stain that keeps coming back | Dry stain from water, nicotine or smoke | Stain-blocking primer |
| Black or green spots | Mould (fungi) | Anti-fungal treatment, then paint |
| Pale streaks on a dark matt colour | Rubbing on the matt film | Washable paint instead of matt |
These are the most likely causes, not the only ones. If a stain grows or returns, get a diagnosis.
Peeling paint
Paint has to grip a sound surface. CIN’s data sheet for Vinylmatt paint (revised July 2026) says that on interior walls already painted you should brush off any old paint that is not adhering, repair damaged areas with filler and then treat the wall as a new surface.
Scrape around the flakes until you reach firmly attached paint. Then:
- if the wall is dusty or crumbles, CIN says to brush it and apply one coat of a primer for friable or powdery surfaces;
- on plasterboard and spray-applied plaster, the same sheet names a dedicated primer;
- on old limewash, CIN says to brush thoroughly and pick the primer according to whether lime is still present on the surface.
The reasoning is simple: on a dusty wall, new paint sticks to the dust rather than to the wall. The primer binds that dust before you paint.
Blisters
Blisters appear when something pushes the paint from behind. Check two causes first.
Water or vapour. A bathroom ceiling with no extractor, a cold wall where moisture condenses, a dripping pipe. Paint fixes none of these. See Damp walls: three types and how to tell them apart and, if the water comes from above, Leak from upstairs: who pays.
Salts. Blisters low on the walls that release white powder when they burst point to salts crystallising between plaster and paint. According to LNEC, Portugal’s national civil engineering laboratory, those crystals build up stresses that break the material apart. The treatment is explained in the guide to salt on walls.
Stains that come through the paint
An old water mark, yellow tobacco staining or soot from a fireplace migrate through fresh paint. Two or three coats of emulsion will not hold them back.
For walls with dry stains from water, nicotine, fireplace smoke or fire, CIN says to brush the surface and apply a stain-blocking primer (in the Vinylmatt sheet, Aqua Primer). That primer’s own data sheet (revised February 2020) adds useful detail:
- the stain must be dry: the product blocks stains, it does not stop water that is still coming in;
- apply 1 to 2 coats, and recoat after 4 to 6 hours;
- below 10 °C and above 80 % relative humidity, the sheet says its stain-blocking ability is reduced;
- the primer only reaches full adhesion after 7 days.
Black or green spots: mould
Dark spots spreading in cold corners, behind furniture or on the bathroom ceiling are usually fungi. Painting over them is not a fix: the mould comes back while the damp continues. First remove the cause (ventilation, condensation, a leak), then treat the surface.
CIN’s data sheet for its Artibiose anti-fungal product (revised August 2026) says not to use bleach to clean the wall before treatment, and not to apply the product if the surface has had bleach in the previous 20 days. It is applied in well-ventilated areas, in two passes, with 24 to 48 hours of drying before the topcoat. For causes and health effects, read the mould on walls guide.
When the painting day is to blame
Even on a well-prepared wall, conditions on the day matter. The Vinylmatt sheet asks for:
- air temperature above 5 °C;
- relative humidity below 80 %;
- a wall 2 to 3 °C above the dew point, meaning a wall not so cold that moisture condenses on it;
- first coat thinned 10 % with water and the following ones 5 %, in 2 to 3 coats.
The same sheet says to wait 28 days before washing a painted wall, and then to rub gently with a soft cloth and neutral detergent. It also warns that matt paints in dark colours can show pale streaks when rubbed. In busy areas, a washable paint is a better choice. See which paint for each room.
Steps to repaint without repeating the problem
- Find the cause. Water, salt, dust, grease, an old stain or the conditions on the day.
- Stop the water, if there is any. Without that, no product will last.
- Remove loose paint back to the firmly bonded layer.
- Clean off dust and grease. Kitchens often have grease on the walls.
- Repair holes and cracks with filler and sand smooth.
- Apply the right primer: binding primer on a dusty wall, stain blocker on a dry stain, anti-fungal treatment on mould.
- Paint in dry, mild weather, with the number of coats in the data sheet. To work out the litres, use the calculator or the guide how much paint do I need.
- Keep the invoice and product names, to work out what went wrong if the paint fails again.
In a quote, ask for the preparation to be spelled out: what gets scraped, which primer, how many coats. See how to compare painting quotes.
What we could not confirm
- Statistics on the most common causes of paint failure in Portugal. We found no official data.
- Repair prices. They depend on how much has to be scraped, the state of the plaster and the cause. See the price guide.
- Manufacturer guarantees. The sheets we read only state that the product meets its technical specification. We did not confirm how a complaint is handled when the wall was not prepared as the sheet requires.
- Other brands’ sheets. For this guide we read CIN sheets only. Other brands may ask for different times and primers: always follow the sheet for the product you use.
Sources (read on 3 November 2026)
- CIN, Vinylmatt 10-250 technical bulletin, revised July 2026 (surface preparation for new, painted, limewashed, stained and mould-affected surfaces; ambient conditions; thinning; coats; notes on washing and streaks on dark colours).
- CIN, Aqua Primer 12-830 technical bulletin, revised February 2020 (uses, preparation, ambient conditions, coats, notes).
- CIN, Artibiose 18-230 technical bulletin, revised August 2026 (surface preparation, application method, notes on bleach and ventilation).
- CIN, Primário Stop Manchas 45-335 technical bulletin, July 2013 (use and notes; confirms the same 10 °C and 80 % limits for stain blocking).
- LNEC, Teresa Diaz Gonçalves and José Delgado Rodrigues, Argamassas de revestimento para paredes afectadas por cristalização de sais solúveis: influência do substrato (“Renders for walls affected by soluble salt crystallisation”), APFAC 2010 (section 1.2, subflorescence and breakdown of the material).
Products are mentioned as examples of public data sheets. ParedeCerta.pt does not sell or endorse any brand.