Mildew is an exterior surface problem and is areas of black, gray or brown spots formed on paint and other surfaces, particularly in damp, shady areas. Mildew is caused by warm, humid conditions, poor air circulation and little direct sunlight, like under eaves or overhangs. Shrubbery planted too close to a building is an ideal place for mildew to grow.
Mildew must be removed before painting or it will come through the new paint. Remove all mildew by scrubbing with a solution of household bleach and water. Mix 1 part bleach to 3 parts water, and remember to rinse the surface thoroughly with a garden hose after washing. (Power washing is recommended for larger areas.) Always wear appropriate safety equipment (rubber gloves and eye protection) when using this solution, and protect your shrubbery and plants with plastic sheeting.
After getting rid of all mildew, priming is essential for better adhesion, sheen uniformity, mildew control and durability. Select a top quality exterior paint in the color and sheen of your choice.