Friday, April 23, 2010

Mildew

The bane of most homeowners, these black, gray, or brown patches on the paint surface are most commonly found on a home's north side, under the eaves, around shrubbery, and on any shady or damp area. Solvent-based, flat, and economy paints are the biggest supporters of mildew growth, but the fungus will feed off of the paint film producing unsightly stain-like blemishes, particularly following an uncommonly rainy season. 

Mildew on painted wood shingle example

Causes:

  • Use of a paint containing little or no mildewcide.
  • Failure to keep the surface clean.
  • Failure to remove mildew.

What  to do:

  • First, test for mildew: Dab (don't rub or smear)  a few drops of household bleach onto the affected area. If the discoloration disappears, you’ve got mildew (otherwise, it's probably dirt, which can be cleaned by scrubbing with a mild detergent). 
  • To remove mildew, scrub the stained surface with a stiff bristle brush and a solution of 1 part bleach to 3 parts water (wear rubber gloves and eye protection and cover nearby plantings and lawn with a tarp). After twenty minutes, rinse well with a garden hose or power washer. Repeated applications may be necessary. On colored surfaces, test a small area first to make sure the bleach solution won't cause color fading. If it does, discuss alternative mildewcides with your paint retailer. Do not paint over mildew! It will continue to grow under the new paint film, creating a far more serious problem.
  • If you need to repaint, prime any bare wood as soon as the surface is dry, and top with one or two coats of exterior latex that has been fortified with an appropriate amount of mildewcide. 

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