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Mildew:
Mildew is the formation of brown, black or gray spots or
blotches on the surface of paint or caulk due to the
presence of fungi that feed on the paint film and other
organic matter.
Possible Causes:
Damp areas that receive little or no direct sunlight
(thus, such areas as walls with a northern exposure and
the underside of eaves are particularly vulnerable).
Use of a lower quality paint with an insufficient amount
of mildewcide.
Painting over a substrate or coating on which mildew is
present.
Failure to prime bare wood before painting.
Solution:
First, make sure that the discoloration really is mildew,
and not just dirt. Apply a few drops of household bleach
to it, wait five minutes, then rinse. If the dark color
disappears, the discoloration is probably mildew. Remove
all mildew by scrubbing with a diluted household bleach
solution (one part bleach, three parts water). Be sure to
take the proper safety precautions -wear rubber gloves and
eye protection. Leave the bleach on the surface for
several minutes, then rinse. Next, thoroughly clean the
surface with a detergent solution to remove dirt and other
organic material on which mildew can feed. This can be
done by hand or with a power washer. Rinse again. When the
surface is dry, apply one or two coats of top quality
acrylic latex paint -- this type of paint contains extra
mildewcide and will resist mildew better than oil-based or
alkyd paints.
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