Mildew......

Mildew is probably the most common cause of the discoloration of house paint, solid -color and semitransparent stains, and natural wood finishes on a home.  Mildew grows on the paint or wood surfaces and does not normally degrade the wood as do wood-rotting fungi.  The most common mildew species are black, but some are brown, red, green and other colors.  Mildew grows most extensively in warm, humid climates.

A simple test for the presence of mildew on paint and a  wood can be made by applying a drop or two of a fresh solution of liquid household bleach ( containing 5% sodium hypochlorite) to the stained area.  The dark color of mildew will usually bleach out in 15 to 30 seconds.  Discoloration that does not bleach out is probably dirt. 

Some wood finishes are more vulnerable than others to attack by ,mildew fungi.   The alkyd and oil-based finishes have more tendency to grow mildew than latex finishes because the resins and oils in these finishes are food sources for the mildew organism.  Mildew progresses more readily on exterior flat house paint than on exterior satin, semigloss, or gloss enamel.  Mildew can be minimized by using topcoats of acrylic latex paints continuing a mildewcide over a primer coat that also contains a mildewcide.

Mildew must be killed and removed before wood is refinished, or the mildew will grow through the new paint coat or solid-color stain.  Removal can be done with commercial mildew remover solutions or with water solutions of liquid household bleach containing sodium hypochlorite.  A common formula for removing mildew from nearly any surface is: 

1/3 cup household bleach( ammonia free)

1 liter 5% sodium Hypoclorite ( liquid household bleach

3 to 4 liters of warm water

This mixture can also be used to remove mildew from naturally finished or unfinished wood such as wood decks.  Warning!!!! Bleach and ammonia combinations are lethal.  People have died from breathing the fumes, similar to mustard gas, from such a mixture.  Many household cleaners contain ammonia, so be extremely careful what type of cleaned is mixed with bleach.

Check back soon for another tip on exterior finishes.

Information taken from Wood Design and Building fall 1999 - Number 9 - Tech Notes Exterior Wood Finishes Part 3: Finish Failures and their Cures.  William C. Fiest, Ph.D.