Updated
Paint Bubbling Or Blistering
Raised bubbles or blisters have appeared in the paint film, sometimes while it was still wet.
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What's happening?
Blistering is a localised loss of adhesion where the film lifts away in a dome while staying intact, rather than flaking off. Something under the paint is expanding or off-gassing and cannot escape through the film. Dulux Trade lists the common causes as too much paint applied in one go, moisture trapped in the plaster beneath, non-heat-resistant paint used near a heat source, and wallpaper below that was hung with too little adhesive — the wet paint softens the paste and the paper lifts, taking the paint with it. Timing tells you a lot: blisters that appear within minutes of painting lining paper usually flatten as the coat dries, while blisters that appear days later have a moisture cause.
What usually causes it?
- Too heavy a coat applied in one pass, so the surface skins before the body of the film has dried
- Moisture still in the plaster, screed or timber below the paint
- Painting in direct sun or onto a hot substrate, so solvent flashes off before the film has levelled
- Emulsion painted over wallpaper that was hung with too little paste, softening the adhesive
- Ordinary paint used behind a radiator, on a flue or near another heat source that needs a heat-resistant coating
- Resin or knot bleed in softwood pushing gas through the film
- Water reaching the back of the film from a defect — a leaking gutter, a failed seal or a live leak
Is it dangerous?
Not dangerous on its own. What it can hide is: a blister that keeps returning in the same place is a moisture path, and left alone that soaks the plaster behind it. Blisters on a boxed-in flue or beside a heat source mean the wrong coating is on a hot surface and should be changed rather than repainted.
Can I fix it myself?
Wait until the coat is fully dry — some blisters in fresh emulsion pull flat by themselves. If they are still there, cut the blister out back to sound paint, fill the resulting depression, sand flush and bring the patch forward with a primer and the same number of coats as the surrounding wall. What you must not do is prick and repaint a blister with a live moisture source behind it, because it comes back in weeks.
When should you call a painter and decorator?
Call a painter and decorator when the blisters keep returning after repair, when they cover a wide area, when they follow a heat source or a suspected leak, or when the wall behind them reads damp. The cause has to be dealt with before the decoration, and identifying which of the seven causes applies is what the decorator's moisture meter and experience are for.
What will the painter and decorator actually do?
- Cut a blister open and read what is behind it — wet plaster, softened paste, resin or a heat-affected film
- Take moisture readings across the affected area and compare with a sound part of the same wall
- Strip the blistered area to a firm edge and let the substrate dry before recoating
- Fill and flat the repaired area so the patch does not read through the new coats
- Specify a permeable coating where damp is present, or a heat-resistant one where the substrate gets hot
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