Mildew forms most often on damp areas that receive little or no direct sunlight. It can also form on walls with a northerly exposure, and on the undersides of eaves.
What Causes It?
Mildew is most often caused by:
• Using lower quality paints, which may have insufficient amounts of mildewcide.
• Failing to prime bare wood before painting.
• Painting over a substrate or coating from which mildew has not been removed.
How to Solve It
You can test for mildew by applying a few drops of household bleach to the area you suspect contains mildew. If it disappears, mildew is likely present.
To remove mildew from a surface, scrub it with a mixture of household bleach and water—one part bleach, three parts water. Be sure to wear rubber gloves and eye protection when you do so.
You can also use power washing to remove surface mildew, though you must still use a bleach solution to completely kill the mildew spores.
After cleaning a surface, rinse it thoroughly and apply primer to any bare wood. Then, apply one or two coats of a Benjamin Moore exterior paint, which contains the proper amount of mildewcide.
Note: images provided by The Rohm & Hass Paint Quality Institute.