Mildew can appear on the surface of paint or caulk as black, gray, or brown spots or areas.
What Causes It?
Mildew most often forms in areas that tend to be damp, or in areas that receive little or no direct sunlight (e.g., bathrooms, kitchens, and laundry rooms). The following conditions can lead to the formation of mildew:
- Using an alkyd, oil-based paint, or
lower-quality latex paint.
- Failing to prime a bare wood surface before applying the paint.
- Painting over a substrate or coating on which mildew has not been removed.
How to Solve It
First, test for mildew by applying a few drops of household bleach to the area. If it bleaches away, the discoloured spots
are probably mildew.
Remove all mildew from the surface by scrubbing it with a diluted household bleach solution (one part bleach, three parts water), while wearing rubber gloves and eye protection. Rinse thoroughly.
To protect against mildew, use a top-quality latex paint, and clean when necessary with a bleach/detergent solution. Also, consider installing an exhaust fan in high-moisture areas.
Note: Images provided by The Rohm & Hass Paint Quality Institute.