What additive can I mix into touch-up paint to extend its mildew resistance on the shaded sides of a New Brunswick home?

Mildew-resistant additives can be mixed into touch-up paint, with zinc oxide and specialized fungicidal additives being the most effective options for New Brunswick's humid Maritime climate.

The most reliable additive for enhancing mildew resistance is zinc oxide powder, which you can mix directly into latex paint at a rate of 1-2 tablespoons per quart. Zinc oxide acts as both a fungicide and UV protectant, making it particularly valuable for those shaded north-facing walls common in NB where mildew thrives in the combination of moisture, moderate temperatures, and limited direct sunlight. You can find zinc oxide powder at art supply stores, some paint retailers, or online suppliers.

Specialized liquid fungicidal additives like Zinsser Mold Killing Primer additive or similar products from Benjamin Moore and Sherwin-Williams offer another option. These are designed specifically for mixing into paint and typically cost $15-25 for a small bottle that treats a gallon of paint. Follow the manufacturer's mixing ratios exactly — too much can affect paint adhesion and finish quality.

For New Brunswick's Maritime climate, this approach makes particular sense because shaded exterior walls — especially those facing north or northeast — stay damp longer after rain and morning dew. The combination of 65-85% summer humidity, moderate temperatures, and limited air circulation creates ideal mildew conditions. Areas under eaves, behind shrubs, or on the leeward side of prevailing winds are especially prone to mildew growth.

Before applying any mildew-resistant touch-up paint, thoroughly clean the existing mildew with a solution of 1 part bleach to 3 parts water, rinse well, and let dry completely. Simply painting over mildew — even with fungicidal additives — traps the growth underneath where it continues to spread and eventually breaks through the new coating.

When to hire a professional: If mildew covers more than 10-15 square feet, or if it keeps returning despite proper cleaning and quality paint, you may have a moisture intrusion problem that requires professional assessment. Persistent mildew on exterior walls can indicate issues with gutters, grading, or building envelope problems common in older New Brunswick homes.

For extensive mildew issues or full-wall repainting, consider having a professional apply a dedicated mildew-resistant primer like Zinsser Perma-White or Benjamin Moore Fresh Start, followed by a high-quality 100% acrylic exterior paint with built-in mildewcide.