How do I fix paint bubbling or blistering on exterior walls of my Fredericton home?

Paint bubbling and blistering on exterior walls is always a symptom of one of two things: moisture trapped behind the paint film, or paint applied in poor conditions that did not adhere properly. In Fredericton, the most common cause by far is moisture — and you need to find and eliminate the moisture source before any new paint will last.

When blisters appear on exterior paint, the paint film has lifted away from the substrate because moisture or vapour has pushed from behind. In older Fredericton homes — particularly wood-framed clapboard and spruce-sided houses built before the 1980s — moisture entry points are plentiful: failed caulking around windows and doors, split or cracked siding, roof leaks that track down inside wall cavities, inadequate vapour barriers in the wall assembly, and even interior humidity from kitchens and bathrooms pushing through walls that lack proper vapour control. The moisture vapour migrates outward and gets trapped under the paint, which expands with the heat of the sun and forms a blister.

The other common cause is paint applied to a wet or damp surface, or in direct hot sun on a very warm day. If the surface temperature gets above 35-40°C, the top skin of the paint cures before the bottom has dried, trapping solvent vapour underneath. In Fredericton's hot July and August sun, south and west-facing walls can reach these temperatures by midday. Blisters from this cause will tend to appear within days of painting rather than months later, and when you pop them, there will be paint — not liquid water — inside.

To fix the problem properly, start by scraping off all loose and blistered paint back to a firm edge or to bare wood. Sand the edges smooth to remove any raised lips. If you find bare wood, probe it with an awl to check for rot — soft or spongy wood needs to be repaired or replaced before painting. Let the area dry completely, which in Fredericton's climate may take several days of dry weather after rain. Test the wood moisture content with an inexpensive moisture meter — you need to be below 15% before any primer or paint can be applied.

Address the moisture source. Caulk any gaps around window frames, door frames, corner boards, and trim joints with a good quality paintable acrylic caulk. Check that gutters and downspouts are directing water away from the wall. If interior humidity is the source, a bathroom exhaust fan upgrade or a dehumidifier can help manage moisture levels.

Once dry, apply a coat of exterior alkyd or shellac primer to the bare spots, feathering out onto the surrounding intact paint. Then apply two full coats of a premium 100% acrylic latex exterior paint — products like Benjamin Moore Aura Exterior or Sherwin-Williams Duration are formulated to be vapour-permeable enough to breathe without blistering, and flexible enough to handle the freeze-thaw cycles Fredericton sees 100+ times a year.

For isolated small repairs, this is a reasonable DIY project if you can identify and fix the moisture source. If blistering is widespread across an entire wall or multiple sides of the house, or if you suspect a structural moisture issue, bring in a professional painter — and potentially a building inspector — to assess whether there is a larger problem inside the wall assembly.

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