Two houses on the same street, painted by the same crew with the same product on the same day, and by August one has blistered and the other looks fine. The difference is not the paint. It is where the blisters are. If they appear only on south- and west-facing walls, and only in the topcoat without exposing bare wood, the cause is heat. If they appear on walls near a bathroom or kitchen, and scraping a blister reveals bare or wet wood beneath multiple layers of paint, the cause is moisture pushing from inside.

Blisters that look identical on the surface can have completely different origins, and the repair for heat blisters is entirely different from the repair for moisture blisters. Treating the wrong cause produces a fresh paint job that blisters again in the next season.

Heat Blisters vs Moisture Blisters on Exterior Surfaces

Heat blisters form when the surface of the paint film reaches temperatures above approximately 90 degrees Fahrenheit while the paint is still wet or during the early stages of cure. At elevated temperatures, solvents in the paint formulation vaporize faster than the film can release them at the surface. The solvent vapor pushes upward through the still-forming film and creates bubbles that freeze in place as the film skins over and traps the vapor. When the trapped vapor eventually dissipates, the blister shell collapses into a loose, empty bubble.

The defining characteristic of a heat blister is that only the most recently applied coat blisters. Scraping a heat blister reveals intact paint layers beneath, not bare wood. The blistering is confined to the topcoat or the last coat applied. The underlying paint remains fully adhered.

Heat blisters are predictably located on the walls that receive the most direct afternoon sun: south-facing and west-facing walls. They are concentrated on areas where surface temperature was highest during application, which is typically the middle height of a wall rather than the shaded bottom or eave-shadowed top.

A second heat blister scenario involves painting over dark existing paint in direct sun. The dark surface underneath absorbs heat, raising the surface temperature of the new topcoat above the threshold even when air temperature seems acceptable. Infrared thermometers that measure surface temperature directly take the guesswork out of this assessment. If the surface reads above 90 degrees Fahrenheit, wait until shade moves across the wall.

Moisture blisters are structurally different. Scraping a moisture blister exposes bare wood or multiple delaminated paint layers underneath. The blister is not a trapped solvent bubble in the topcoat. It is the paint film separating from the substrate because water has accumulated at the paint-wood interface and the vapor pressure beneath the film has exceeded the film’s adhesion.

Moisture blisters appear on walls regardless of sun exposure. They concentrate near bathroom exhaust vents, laundry room walls, kitchen walls, and any location where interior humidity is pushed through the wall assembly. They also appear on walls where exterior caulk has failed and is allowing rain to penetrate behind the siding, and on low walls that receive consistent ground splash saturation.

A third blister type, solvent entrapment, occurs when oil-based paint is applied directly over a latex base coat. The oil film traps solvents from the latex below as the oil film cures and becomes less permeable. These blisters appear regardless of weather conditions and are identifiable by their occurrence on surfaces that received incompatible paint layers.

Identifying the Source of Blistering Before Repainting

Correct identification of the blister source is not optional before repainting. Repainting over the original cause without addressing it produces an identical result.

For suspected heat blisters, note the exact wall orientation and height where the blistering is most concentrated. Compare this to the sun exposure pattern. Heat-blister patterns follow solar paths predictably. Blistering on a north-facing wall is not heat blistering.

For suspected moisture blisters, identify the rooms adjacent to the blistered wall on the interior. High-humidity rooms such as bathrooms, laundry rooms, and kitchens are the primary sources of moisture that migrates outward. Inspect the wall exterior for failed caulk joints, missing flashing, and any point where water could be entering from outside. Use a moisture meter on the siding boards at the blister locations. Readings above 15 percent confirm active moisture at the surface.

Poke or scrape one or two blisters to confirm the type. Bare wood beneath the blister is moisture-driven. Intact paint beneath the blister is heat-driven. If the wood beneath the blister is soft or darker in color than surrounding areas, the moisture has been present long enough to begin wood degradation.

For oil-over-latex solvent entrapment blisters, check the previous paint history. Homes repainted multiple times over decades sometimes accumulate incompatible layers. A simple paint type test using denatured alcohol: if the paint dissolves with alcohol, it is latex. If it does not, it is oil-based.

How to Repair Blistered Areas Without Stripping the Entire Surface

Heat blisters that reveal intact underlying paint layers can be repaired without stripping the entire surface.

Scrape all blistered areas to remove the loose, bubbled topcoat. Sand the scraped areas with 100 to 120 grit sandpaper to smooth the paint edge and feather the transition between the scraped area and the intact surrounding paint. Apply primer to the sanded bare areas and allow to dry completely.

Apply the topcoat in the repair area. Select the time of day for this application carefully. East-facing walls: apply in the afternoon when the morning sun has moved off the surface. West-facing walls: apply in the morning before afternoon sun hits. South-facing walls: apply in early morning or late afternoon when the sun angle is low and surface temperatures are below 90 degrees Fahrenheit.

For large heat-blistered areas, consider reformulating the approach for that wall. If the south-facing wall blistered because the surface was above 90 degrees Fahrenheit on the application day, repainting the same wall at the same time of day with the same product will produce the same result. Commit to a different application schedule for that elevation.

Moisture blisters require resolving the moisture source before any paint repairs are attempted. Repainting over a moisture-blistering situation without stopping the moisture guarantees re-blistering. The repair sequence is: identify and eliminate the moisture source, allow the wood to fully dry (moisture meter below 15 percent), scrape all blistered areas, sand feather edges, prime with a moisture-blocking primer, and topcoat.

Moisture sources require different fixes. Interior humidity migrating through walls: improve ventilation in the source room, add or repair exhaust fans, and consider adding a vapor retarder in the wall assembly. Exterior water entry: replace failed caulk, repair or install flashing, ensure gutters direct water away from the wall. Ground saturation at foundation: improve grading to direct water away from the foundation, extend downspouts further from the house.

After the moisture source is controlled, allow the wall to dry for a minimum of two to three days of fair weather before repainting. Use a moisture meter to confirm readings below 15 percent.

Preventing Blistering on Future Exterior Paint Jobs

The conditions that produce heat blistering are avoidable with schedule management. Surface temperature is a measurable quantity. An infrared thermometer costs less than $30 and provides a direct surface temperature reading in two seconds. Check every wall before starting to paint it. Do not paint any surface above 90 degrees Fahrenheit, regardless of air temperature or apparent weather conditions.

The surface scheduling principle: paint each wall face after the direct sun has moved off it. East walls in the afternoon. West walls in the morning. North walls at almost any time of day. South walls early morning or late afternoon when solar angle is low.

Avoid painting in late afternoon when temperatures are dropping rapidly. A surface that was at 80 degrees Fahrenheit at 3 PM drops into the dew point risk zone by 8 or 9 PM. Paint applied three to four hours before a surface cools to the dew point temperature is at risk of overnight moisture condensation on the partially cured film.

For moisture blister prevention, the key technical controls are: wood moisture content below 15 percent at application, surface temperature at least 5 degrees Fahrenheit above the dew point, all exterior caulk joints sealed before painting, and adequate ventilation in high-humidity interior spaces.

Choosing paint colors with Light Reflectance Values above 40 on sun-facing walls reduces thermal loading. Lighter colors absorb less heat and keep surface temperatures lower during the critical initial cure period. Dark exterior colors on south- and west-facing walls carry inherently higher blister risk in hot climates, particularly when applied during warm months when ambient temperatures are already high.