University of Wisconsin - Extension

If your house is heavily shaded or near a lake, you may have noticed dark spots on your siding, according to a University of Wisconsin-Extension housing specialist.

¿Dark spots are often a sign of mildew on your wood siding,¿ says John Merrill. ¿Even houses that aren¿t shaded or near water can develop this problem.¿

On hard board siding, wax bleeding through can occur if moisture is forced through siding. In cedar and redwood siding, resin bleed through can cause dark spots.

If you think the spots are caused by mildew, apply a drop of household chlorine bleach. Bleach should cause mildew to lighten almost immediately.

Mildew can be a sign of more serious moisture problems, such as excessive indoor humidity or a leaking roof. If you find soft or swollen siding in addition to the mildew, look for the underlying moisture problem before treating the mildew.

In many cases, mildew will grow without underlying moisture problems. Conditions leading to mildew include nutrients, such as dust, paint or stain on the siding, combined with moisture and the right temperature. However, moisture is often the critical factor in determining if mildew starts growing.

Shade and high humidity can create moisture that condenses on the siding instead of evaporating, providing mildew with the moisture it needs.

Occasionally, hosing down the siding with water and a soft scrub brush will keep the food supply down, reducing the chances of mold growth. It will probably make the siding look better as well.

Once mold develops, washing the surface with a bleach solution usually removes mildew. The Paint Quality Institute recommends a solution of three quarts water to one quart chlorine bleach. Use a garden sprayer to wet the siding with the bleach solution and allow it to remain on the siding for about half an hour, then rinse thoroughly.

Be sure to rinse landscape plants before applying the bleach or cover them with clear plastic during the application process. Remove the plastic as soon as the application is complete to avoid heat damage to the plants. Read the instructions on the bleach container. Eye or skin contact with bleach can cause problems.

If you are planning to repaint your siding, it is essential that you remove all mildew before you start. If you don't, it is likely to grow right through the new paint.

The Paint Quality Institute recommends using a latex paint as opposed to an oil or alkyd paint. Latex paints are less likely to support mildew growth. A satin formula will be more mildew resistant than a flat formula because it is less likely to collect debris from the air.

Paints usually contain some mildewcide, however, top quality paints may have more. You can also add mildewcides to paint. If mildew is a special problem on your house, ask your paint supplier if adding additional mildewcide is a good idea for the paint you have selected.

For areas that require scraping to bare wood once the mildew is removed, begin by applying a latex primer compatible with the top coat paint you select. If there are large areas of bare wood, begin by coating the bare wood with a water repellent preservative before the primer to produce a longer lasting paint coating.

If you are using a semi-transparent stain, an oil-based product still appears to be the best bet. However, if mildew is a problem, be sure the stain you use includes an effective preservative or mildewcide.