Three difficult exterior issues that affect paint appearance and longevity are all caused by excessive moisture. Minimizing the moisture problem first and then renewing the substrate with quality paints/coatings is the key to success.
Mildew is an unsightly fungus that can grow on many surfaces including paint. The mildew feeds on nutrients available in both the paint film and the underlying surface. Most important for mildew growth is moisture, so it is commonly found in damp, dark places frequently under eaves and behind shrubbery.
Mildew must be removed prior to painting. Obtain a mold biocide, which must include a surfactant agent (soap) and a mold killer (commonly on exterior substrates bleach is used). Active, dead and hibernating mold spores need to be removed this is where the surfactant helps clean the substrate for priming/painting. Scrub the area thoroughly while protecting yourself by wearing rubber gloves, eye protection and other protective clothing. Don’t allow the biocide to dry out on the surface. Rinse the area thoroughly and allow the area to dry. Prime any exposed substrate as necessary and repaint the area as soon as possible.
Allowing more sunlight and better air movement to reach the affected area will help minimize the return of the mildew, so pruning trees and thinning shrubbery are important steps to prevent/control potential mold growth.
Source: Painting Pro Times, 4/13/16. Author: Matt Ferring. Moisture – The Exterior Curse