Why is my paint bubbling or blistering?
A blister is either water pushing from behind or solvent trapped in front. Burst one and look underneath — the answer decides whether this is a repaint or a building repair.
- Difficulty
- intermediate
- Time
- 30 min
- Read
- 5 min
- Safety
- caution
Short answer
Burst a blister and look at the substrate beneath. Damp or stained means moisture behind the film, and the source has to be found and stopped before any repainting. Dry means the coat skinned over before it could release — too thick, too hot, or over a warm surface — which is a preparation and repaint job.
Blistering looks like one fault and is two, with opposite causes and opposite repairs. In one, water behind the coating builds enough vapour pressure to lift it off the wall. In the other, the surface of the coating sets before the solvent or water inside it has escaped, and the trapped vapour pushes the skin outwards. Getting this the wrong way round is why people redecorate the same wall three times.
Safety
What you'll need
- A sharp knife or scraper, to open a blister
- A torch, and ideally a moisture meter
- Filler and abrasive paper, for making good
- The appropriate primer for whatever you find
Step by step
- Cut a blister open and look underneath.Slice it cleanly and lift the flap. Is the substrate damp, discoloured or stained, or is it dry and clean? This is the whole diagnosis and it takes seconds. Do it in two or three places in case they differ.
- Read the timing as well.Blisters that appeared within hours of painting are almost always solvent or heat blisters. Blisters that appeared weeks or months later, or that come and go with the weather, are moisture. This confirms what you found under the flap.
- Read the pattern.Moisture blisters cluster: below a window, around a chimney breast, on one external wall, in a line along a ceiling, or on the coldest surface in a room. Solvent blisters follow where you painted thickest or where the sun was on the surface. A pattern that maps onto the building rather than onto your brushwork means water.
- If it is moisture, stop decorating and find the water.Check above the affected area for a leaking pipe, a failed shower seal, a slipped tile or a blocked gutter. Check external walls for defective pointing, a bridged damp-proof course, or ground level raised above it. Check for condensation on cold surfaces. Repainting a wet wall traps the water and enlarges the eventual repair.
- If it is solvent or heat, work out which of the three causes applies.Applied too thick, so the surface skinned before the body of the film could release; applied in direct sun or on a hot substrate; or applied over a surface that was subsequently heated, such as a radiator wall or a south-facing door. Each is avoidable by changing when and how you apply the next coat.
- Cut back to sound material and rebuild.Scrape out all the blistered material and keep going until the coating genuinely will not lift. Feather the edges, fill any hollows, prime as the exposed substrate requires, and rebuild the coats — thinner ones, in cooler conditions, on a surface that is dry.
- Let a repaired damp area dry before you close it up.Once the leak is fixed, plaster and masonry take a long time to give up their moisture — weeks to months depending on the wall. Painting too soon repeats the failure. Use a moisture meter or simply wait until the area behaves like the rest of the wall.
Tips
- Blisters on the outside of a window frame at the bottom rail are end grain drawing water in. That is a joinery repair, not a paint problem.
- Small blisters over a filled repair are often the filler still holding water. Filler needs to dry through before it is painted, and deep fills take much longer than they look.
- A blister that reappears in exactly the same spot after a good repair is the building telling you where the water is. Trust it.
Common mistakes
- Sanding blisters flat and painting over them — If the cause is moisture, it lifts the new coat too. If the cause is trapped solvent, the void is still there under a thin skin and will break through.
- Assuming a bathroom or kitchen blister must be condensation — It is just as likely to be a leaking seal, a failed waste connection or a pipe in the wall. Look before you conclude.
- Repainting a damp wall because it looks dry on the surface — The surface dries first and the depth of the wall dries last. A meter or a taped polythene patch tells you what the surface cannot.
If it doesn't work
Blisters full of water
Cause: An active leak or serious penetrating damp — Fix: Find and stop the water first. Isolate any electrics in the affected area, trace the source above and around, and only decorate once the wall has dried out.
Blisters on a south-facing door, appearing the day it was painted
Cause: Sun on a dark surface, or a coat applied too thick — Fix: Sand back, prime bare areas, and recoat in thinner coats early or late in the day, out of direct sun.
Blisters on the wall directly above a radiator
Cause: Heat driving moisture and solvent through a coat applied on a warm wall — Fix: Cut back, turn the heating off, let the wall cool fully, then prime and recoat.
Blisters on the outside of exterior masonry after rain
Cause: Impermeable coating trapping water in the wall — Fix: This is a coating-choice problem. Remove the failed film and use a vapour-permeable system.
Questions people ask
Can I just pop the blisters and fill them?
Only if they are dry solvent blisters and you have cut back to genuinely sound material. On a damp wall, filling a blister seals water in and the fill will fail.
Why did my paint blister within an hour of applying it?
Almost certainly heat or thickness: a hot substrate, direct sun, or a coat laid on far too heavily so the surface skinned before the film could release its water or solvent.
How long should I wait before repainting a wall that had a leak?
Until the wall is genuinely dry through, not just at the surface — often weeks, sometimes months for a solid wall. A moisture meter or a taped polythene square is worth more than a guess.