What is the best way to deal with mildew on exterior siding before repainting in Moncton?

Mildew must be completely killed and removed before repainting — painting over active mildew is one of the most common and costly exterior painting mistakes in Moncton, and it causes new paint to fail within a single growing season. Mildew is a living organism that grows right through new paint, and the dark stains it leaves will bleed through fresh coats even after the mildew is dead unless you treat the surface properly.

Moncton’s combination of Maritime humidity (regularly 65-80% relative humidity in summer), moderate temperatures, and significant tree canopy in many residential neighbourhoods creates near-ideal conditions for mildew growth on north-facing siding, soffits, fences, and areas shaded by trees or neighbouring structures. The dark grey-to-black discolouration you see on these surfaces isn’t just dirt — it’s biological growth (a mix of mildew, algae, and sometimes moss) that needs to be addressed differently than ordinary cleaning.

The proper treatment process:

Start with a bleach-water solution: mix 1 part household bleach (sodium hypochlorite) with 3-4 parts water and add a small amount of dish soap or a purpose-made exterior cleaner like Jomax. Apply with a garden sprayer or soft-bristled brush, working from the bottom up to avoid drip streaks setting into dry siding. Let the solution dwell on the surface for 10-15 minutes — you’ll often see the black colouration visibly lighten as the bleach kills the mildew.

Rinse thoroughly from top to bottom with clean water. A pressure washer at 1,000-1,500 PSI works well for rinsing; on softer or older wood siding, use a garden hose with a high-pressure nozzle rather than a pressure washer, which can raise the grain and damage the surface.

Allow the siding to dry completely. In Moncton’s climate, this means a minimum of 48-72 hours of dry weather before priming. Check with a moisture meter — wood siding should read below 15% moisture content before paint goes on. Rushing this step on a damp NB spring day is the reason mildew problems recur.

Prime with a mildew-resistant primer. Many exterior primers contain mildewcides — check the label for this claim and use it on previously affected surfaces. Stain-blocking primers like Zinsser Bulls Eye or a quality oil-based exterior primer will also help prevent bleed-through of residual mildew staining.

For the finish coat, choose an exterior acrylic latex with built-in mildewcide — Benjamin Moore Aura Exterior and Sherwin-Williams Duration both contain mildewcides in the formulation. These don’t prevent mildew indefinitely (nothing does in Moncton’s climate), but they significantly slow regrowth.

If mildew is a recurring annual problem on specific areas of your home — especially north faces or areas under large trees — address the conditions: trim branches to increase sun exposure and airflow, check eavestroughs for drips onto the siding, and ensure the grading around the foundation directs water away from the house. Annual light cleaning with a diluted bleach solution in spring will extend the life of your paint job considerably.

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