How do I remove mildew from exterior wood before painting a home in coastal New Brunswick?

Mildew on exterior wood must be killed and removed before painting — never simply painted over. Paint applied over mildew creates a sealed, warm, slightly moist environment that mildew loves, and it will grow right through the new paint within a season, leaving black or grey stains on your fresh finish.

This is an especially important issue for homes along the Bay of Fundy coast, around Shediac, Bathurst, and the Saint John harbour area. The combination of salt-laden air, prevailing moisture off the ocean, and shade on north-facing walls creates perfect mildew conditions. Even inland homes in the Saint John River valley and around Fredericton deal with mildew on soffits, protected overhangs, and the lower courses of siding that stay damp after rain.

The standard treatment is a diluted bleach solution: mix one part household bleach (sodium hypochlorite) with three parts water and add a small squirt of dish soap to help the solution penetrate and cling to the surface. Apply with a stiff brush or pump garden sprayer, scrub the mildewed areas thoroughly, and let the solution sit for 10-15 minutes. The bleach kills the mildew organism — not just the visible stain, but the fungal roots embedded in the wood. Rinse thoroughly with clean water and let the surface dry completely. For stubborn mildew on older, weathered wood, a second application may be needed.

There are also commercial mildew removers and exterior cleaners available at NB paint stores and hardware stores — products containing sodium hypochlorite or sodium percarbonate work on the same principle as the bleach solution and are sometimes easier to apply safely. Expect to pay 5-30 for a concentrated mildew treatment product that covers a significant area.

Safety matters during this process: wear rubber gloves, safety glasses, and old clothes — bleach splatter is unpredictable, especially when scrubbing or using a sprayer. Don't mix bleach with any other cleaning products, including ammonia-based cleaners, which creates toxic chloramine gas. Keep children and pets away from the work area while cleaning and rinsing.

After cleaning and rinsing, give the wood at least 48-72 hours of dry weather to return to safe moisture levels before applying primer. Test with a moisture meter — wood needs to be below 15% moisture content before painting. In NB's Maritime climate, shaded north walls can hold moisture for longer than you might expect even after they look dry on the surface.

Once dry, prime with an exterior primer that contains a mildewcide — many quality exterior primers and paints include mildewcide additives (Sherwin-Williams Exterior Primer, Benjamin Moore Fresh Start Exterior Primer, and Dulux's exterior line all have mildewcide options). The mildewcide doesn't eliminate the need to clean first, but it inhibits regrowth in the new film. Finish with a 100% acrylic latex exterior paint, which breathes better than oil-based coatings and resists mildew colonisation far better in NB's humid conditions.

For homes where mildew covers large areas or keeps coming back aggressively year after year, the underlying cause may be a ventilation issue in the attic or wall cavity. A professional painter can help identify the pattern, but if the mildew is always in the same spot despite proper cleaning and painting, a building envelope assessment may be worthwhile.

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