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Q.Which paints are more mildew-resistant, oil-based or acrylic? How can I prevent mildew from growing on painted surfaces?

A.Bill Feist, formerly a research chemist at the USDA Forest Products Laboratory in Madison, Wis., responds: Some paints are more vulnerable than others to attack by mildew fungi. For example, with most types of paint, mildew grows more quickly on exterior flat paints than on exterior semigloss or high-gloss enamels. Also, alkyd and oil-based paints have more of a tendency to grow mildew than acrylic latex paints because the resins and oils in these paints are food sources for the mildew organisms. Of the available water-based paints, acrylic latex is usually considered the most resistant to mildew.

However, this doesn’t mean that acrylic paints will resist mildew indefinitely. In most climates, any paint, whether acrylic or oil-based, that contains a mildewcide will eventually support mildew growth as the mildewcides break down. For exterior wood, you should always use paints and primers that contain a mildewcide (most high-quality exterior paints are formulated with mildewcides). In hot, humid climates, where the mildew problem can get quite severe, it’s probably a good idea to have the paint store mix in extra mildewcide, such as M-1 Additive, made by the Jomaps Co. (6500 Industrial Way, Alpharetta, GA 30201; 770/442-8808) and sold in Sherwin-Williams stores.