Mould

Black, grey or brown areas on the painted surface.

Possible Cause

• Forms most often on areas that tend to be damp, and receive little or no direct sunlight (the underside of eaves are particularly vulnerable).
• Use of a lower quality paint.
• Failure to prime bare wood before painting.
• Painting over a substrate or coating on which mould has not been removed.

Solution

Test for mould by applying a few drops of household bleach to the discoloured area - if it disappears, it is probably mildew. Remove all mildew from the surface by scrubbing with a diluted household bleach solution (one part bleach, three parts water); wear rubber gloves and eye protection. Power washing is also an option. Rinse thoroughly, prime any bare timber, then apply one or two coats of top quality exterior paint.

Images courtesy of the Paint Quality Institute from Dow Coating Materials