Mildew on Exterior House Paint

Mildew on Exterior House Paint

What is mildew?

Mildew is a common discoloration on the surface of house paint.  Mildew is a surface-fungi can easily be identified as a patch of gray or even white fungi on the surface of a moist area.  It will not damage the siding, but it will decrease the aesthetics of your house.   Mildew fungi are most common in warm, humid climates. 

Mildew is easily treated with any retail-purchased cleaner and a scrubbing brush.  I annually use a pressure washer at my house to remove mildew from my Masonite siding.  It is important to apply enough pressure to remove mildew but not too much so that the paint is removed. 

Whatever the type of siding, algae, mold and mildew are most likely to form on the north side of a house because it stays shaded and damp.  However, mildew can grow on all sides of a house, particularly on walls behind trees or shrubs where movement of air is restricted.

Is it mildew or dirt?

Mildew is often confused with dirt.  Both are common surface discolorations.  However, they can easily be distinguished. 

  • Mold likes to grow in areas of high humidity and little air flow. Consequently, an air conditioning duct wouldn’t normally be a place for mold because there’s lots of air flow and low relative humidity.
  • Mold needs something on which to grow. If you’re looking at something that’s steel or aluminum, it’s probably not mold.
  • Mold usually doesn’t wipe clean. It tends to grow into whatever it’s growing on.  If you dampen a cloth and can rub everything off, it probably isn’t mold.
  • Mold has a velvet-like appearance that can be noticed upon close inspection. Also, it tends to vary in color intensity from bright to dull.
  • Mold will usually have an odor, especially when the room ventilation system is shut off. Most molds have a “musty” smell or a basement smell when there’s little or no air flow.  The odor is caused by the waste products of mold growth.  If possible, close off the room for a day and see if you detect any odor.
  • Mold doesn’t like lighted areas. If you’re looking at something that’s in a well-lighted area, chances are it isn’t mold.

A useful confirmatory test for the presence of mildew on paint can be made by applying a drop or two of common household bleach solution (5% sodium hypochlorite) to the stain.  Mildew will usually bleach out in 1-2 minutes.  A stain that does not bleach out is probably dirt.  It is important to use fresh bleach solution because bleach deteriorates on standing and loses potency. 

Does Bleach kill mold?

Yes, but it comes with a catch. Bleach labels will warn you that chlorine bleach will only be effective on a “hard, non-porous surface.’’ This basically means that chlorine bleach is not made to “soak in.” Therefore, its disinfecting properties are limited to a hard surface like tile or glass. So, here’s the problem: To ensure survival, mold spores spread its roots (mycelia) deep into a porous surface. Mold remediation requires a cleaner to reach deep down into wood and other porous building materials to remove or “pull out” the roots. The properties of bleach prevent it from soaking into these materials. The surface mold looks gone (it’s bleached white) but the internal mold always remains to grow back.

Another issue: Bleach contains 90% water and mold loves water. When bleach is applied, the chlorine quickly evaporates after use leaving behind water. This water often soaks into the porous surface allowing the mold to flourish and re-grow in this moist environment. In effect, using bleach actually feeds the internal mold spores. Although the surface may look bleached and clean, the remaining spores will root deeper, stronger and will often return worse than before. 

Prevention and Cure

Use a paint containing zinc oxide and a mildewcide for top coats over the mildewcide-containing primer coat.  If you are unsure if your new paint has a mildewcide in it, you can purchase a mildewcide from a paint store and mix it into the paint.

If dealing with siding that has mildew, begin by removing the mildew.  Then, apply a water-repellent preservative or other mildewcide before repainting. 

There are many mildewcide products on the market.  I have used many at my house and conducted laboratory research on many more products.  The product that does the best job is Bora-Care® with Mold Care®.   This is a very safe product that can be used inside and outside the house.  It not only removes and prevents mold and mildew, but it also adds additional residual protection against decay fungi and insects.  This product penetrates the wood and protects wood from the inside out.  Other products do not have these attributes.

Meet the Author

Dr. Todd Shupe is the President of Wood Science Consulting, LLC. He is a well-recognized expert on wood forensics, wood preservation, wood decay and degradation, and wood species identification. He has a broad background in new product development, quality management, and marketing and sales in both the public and private sectors. For more information please visit

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