How do I prevent mildew from growing on painted exterior surfaces in humid NB summers?

Mildew on exterior painted surfaces is one of the most common complaints from NB homeowners, and with good reason — the combination of Maritime humidity, shade from mature trees, and warm summers creates nearly ideal conditions for mildew and algae growth on siding, soffits, and trim. Prevention starts with the right paint choice and continues with regular maintenance.

Mildew is a living organism, and it grows wherever it finds moisture, organic material to feed on, and moderate temperatures. North-facing walls that stay damp longest, shaded sides of the house where a neighbour's trees block the sun, and soffits over porches where air circulation is poor are the classic mildew hot spots in NB. River valley communities like Fredericton along the Saint John River, and coastal communities from Shediac to Saint John, see heavier mildew pressure than drier inland areas, but really no part of New Brunswick is mildew-free during July and August.

Choosing a paint with a mildewcide built in is your first line of defence. Most premium exterior paints — Benjamin Moore Aura Exterior, Sherwin-Williams Duration, Dulux Weathershield — contain mildewcide compounds that inhibit surface mould and mildew growth. These are not permanent — the mildewcide concentration at the paint surface diminishes over time through weathering — but they provide meaningful protection for the first several years after a fresh coat. Budget exterior paints often contain little or no mildewcide, which is one reason you see mildew appearing within one or two seasons on cheaper paint jobs. When getting quotes for exterior painting in NB, ask specifically whether the proposed product contains a mildewcide.

Surface prep before repainting is the most important prevention step. Never paint over mildew. Mildew that is painted over is not killed — it is sealed in a moist, dark environment that is actually more hospitable than the open air. It will grow right through the new paint within one season. Before any exterior repaint, wash mildewed surfaces with a solution of one part bleach to three parts water, scrub with a stiff brush, and rinse thoroughly. Let the surface dry for several days before priming and painting. TSP cleaner added to the wash solution helps remove dirt and organic material that mildew feeds on.

Ongoing prevention between repaints: an annual spring cleaning with a diluted bleach solution or a commercial mildew wash product (available at paint stores across NB) removes spores before they can establish colonies. A low-pressure garden hose rinse of north-facing walls and soffits after prolonged humid periods helps too. Trim back any trees or shrubs that press against siding or shade walls for most of the day — improving air circulation and sunlight exposure is one of the most effective long-term mildew controls available to homeowners.

For surfaces that are chronically problematic — a north-facing wood wall that shows mildew every two to three years regardless of how well it was painted — consider asking your painting contractor to add a mildewcide additive (like Perma-White additive or a commercial product) directly to the paint at mixing time, over and above what the paint already contains. This is a legitimate practice that provides an extra layer of biological protection for persistently problematic surfaces.

If mildew is recurring rapidly on the same surface despite proper prep and quality paint, it often signals a moisture issue beyond just humidity — a drainage problem at the foundation, a gutter that is overflowing, or siding that is holding moisture from behind. A professional painter can often spot these patterns during an exterior assessment and recommend structural fixes alongside the paint solution.

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