Why Is My Exterior Paint Blistering? Cut One Open

Short answer

Paint blisters for one of two reasons, and cutting one open tells you which. If there is bare wall under the blister, often damp, or water comes out, it is a moisture blister: water is getting into the wall or through it from inside and pushing the paint off from behind. That will keep happening until the water source is fixed. If there are older coats of paint under the blister, it is usually a heat or solvent blister: the top coat was applied in direct sun, on a hot surface, or too thick, and skinned over before it could dry. That is an application problem, fixed by scraping, sanding, and repainting in the right conditions. Either way, painting over the blisters does not work.

A FAIME painter working the upper wall of a commercial building from scaffold, beside masked windows, in Vaughan — Working the upper wall from scaffold on a FAIME commercial repaint in Vaughan. Conditions on the day of painting decide whether a coat blisters.

The one test that matters: cut a blister open

Slice, peel, look underneath

Pick a blister, slice it across with a sharp utility knife, and peel back the skin. Bare wall underneath (stucco, wood, masonry or primer), especially if it feels damp or water comes out, means moisture is pushing the paint off from behind. An older coat of paint underneath, dry, means the blister formed in the newest coat as it dried. Check a few blisters on different parts of the wall. Occasionally both are going on at once.

Moisture blisters

Bare, often damp wall under the blister
Concentrated in one area: below a window, a roof line, a gutter, near the ground, or on the outside of a bathroom, kitchen or laundry room
Come and go with the weather or appear after rain
Can appear on paint that is years old

Heat or solvent blisters

Older paint under the blister, dry
Appear within hours or days of painting
Spread over sunny elevations painted on a hot day
Often small and many, across the whole area painted that day

Where moisture blisters come from

A moisture blister is the wall telling you it is wet. The water has to come from somewhere:

From outside: failed window caulking or joint sealants, cracked stucco, a missing or failed flashing, an overflowing gutter, a downspout discharging against the wall, or splashback and ground moisture at the base.
From inside: warm, humid air from bathrooms, kitchens, laundry rooms and pools moving out through the wall and condensing behind the paint. Blisters on the outside of those rooms, and not elsewhere, point here.
From the wrong coating: a paint or sealer that does not let the wall breathe, applied over masonry, stucco or parging, traps moisture that used to evaporate from the face.

Until the water source is fixed, new paint on that wall blisters again, usually in the same place.

Why heat and solvent blisters happen

Paint dries from the surface down. If the surface skins over too fast, whatever is still evaporating underneath gets trapped and pushes the new film up into bubbles. That happens when:

It is painted in direct sun or on a hot surface. A dark wall in afternoon sun can be far hotter than the air temperature. The standard practice is to follow the shade around the building.
The coat is too thick. A heavy coat skins over before it has dried through.
The surface was damp. Dew, a recent rain or a wall that was washed and not given time to dry leaves moisture under the new film.
Rain or heavy dew arrives before the paint has cured.

How to fix blistering paint

Diagnose first

Cut blisters open in several places. Moisture blisters send you looking for the water. Heat blisters send you back to how and when the wall was painted.

Fix the water source

For moisture blisters: repair the caulking, joints, stucco, flashing or drainage letting water in, and deal with interior humidity where that is the cause.

Scrape, sand and let it dry

Remove every blister and any loose paint around it, feather the edges so the repair does not show, and give the wall time to dry out completely.

Prime and repaint in the right conditions

A primer suited to the surface, then finish coats at normal thickness, in shade, on a dry wall, with a dry forecast. On masonry and stucco, a coating that lets the wall breathe.

Where FAIME fits

Exterior painting is a self-performed FAIME scope, together with the caulking, sealant and stucco repair work that moisture blisters usually need first, on houses, storefronts and large commercial buildings across Toronto, North York, Scarborough, Vaughan, Richmond Hill, Markham, Mississauga, Brampton and the wider GTA.

On our Vaughan commercial envelope project, the repairs, sealant joints and window caulking were all done before two full coats of paint went on, which is the order that stops moisture blisters coming back. If your paint is flaking rather than bubbling, read why exterior paint peels.

$2M general liability · Active WSIB (Class G5) · 2-year workmanship warranty
Sherwin-Williams, Benjamin Moore and Dulux coatings

Frequently Asked Questions

Why is my exterior paint bubbling?

Either moisture is pushing the paint off from behind, or the top coat trapped heat or solvent as it dried. Cut a blister open: bare, often damp wall underneath means moisture; older dry paint underneath usually means a heat or solvent blister from painting in direct sun, on a hot or damp surface, or too thick.

Can I just pop the blisters and paint over them?

No. The paint around a blister has already lost its bond, and with moisture blisters the water is still getting in. Scrape and sand off every blister and any loose paint, fix the water source if there is one, let the wall dry fully, prime, then repaint in the right conditions.

Will paint blisters go away on their own?

Heat blisters sometimes flatten a little as the paint cures, but the film has still lost its bond and usually cracks and flakes later. Moisture blisters come back for as long as water keeps reaching the back of the paint.

Why is paint blistering only on the outside of my bathroom?

That pattern usually points to moisture from inside: warm, humid air moving out through the wall and condensing behind the exterior paint. Better ventilation inside is part of the fix, along with repairing and repainting the outside with a coating suited to the wall.

What temperature is too hot to paint outside?

It depends on the product, so follow the manufacturer's range, but the bigger risk is surface temperature, not air temperature. A wall in direct afternoon sun can be far hotter than the air. Painting in shade and following the shade around the building avoids most heat blistering.

Does FAIME repair blistering exterior paint in the GTA?

Yes. Exterior painting, and the caulking, sealant and stucco repairs that moisture blisters usually need first, are self-performed FAIME scopes across Toronto, Vaughan, Richmond Hill, Markham, Mississauga, Brampton and the wider GTA. FAIME is WSIB Active under Class G5, carries $2M general liability and backs its workmanship with a 2-year warranty. Site visits are free.

Why Is My Exterior Paint Blistering? Cut One Open and Look