Adding mildewcide to exterior paints does not make it mildew-proof. Mildew thrives in an environment with moisture, humidity, and shade. As mildew is an airborne fungus, it grows primarily on organic material. If there is surrounding vegetation , grass or leaves may blow onto siding, and mildew will grow on that material. In some climates or environments, it may be necessary to clean exterior surfaces with a bleach solution every six months or so.
Exterior paints have mildewcide in their formulations, which are regulated by the EPA. This prevents mildew formation on the paint film, but again, does not make the paint mildew-proof. Alkyd (oil-based) paints , in comparison, attract mildew because the alkyd resin provides is organic.
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