Mold or mildew on siding is particularity prevalent if your home is shaded or near a lake. These fungi are often mistaken for dirt when attached to your siding. If you fail to remove it and paint over it, the mold will come back. If you don't need to repaint, you may have grown weary of the appearance and just want it gone. Homeowners have a couple of options.
The Food Chain
Mold and mildew are types of fungi. Mold is typically black or green, and mildew tends to be gray or white. The growth of mold and mildew is the result of spores in the air, attaching to the surface of objects. Mildew and mold feed on wood, as well as dust and debris attached to your siding. Shrubs and trees produce sap -- you might have noticed it on your car's windshield -- mold and mildew find these sugary solutions delicious. Mold and mildew management is two-fold: the removal and then prevention of a re-occurrence by keeping the food source at bay.
Oxygen bleach is one of the safest ways to remove mold from natural wood or wood-based siding. This product -- not to be confused with chlorine bleach -- is a granulated powder that mixes with water. Follow the instructions for the mixture, and apply it with a pump sprayer in workable sections. Allow it to remain on the surface for 15 minutes. Scrub the mold and mildew off using a nylon scrub brush or broom, starting from the top to prevent streaks. Then rinse. It's a good idea to mix the solution weaker than recommended and to test it on a small area. Increase the concentration as needed until you find a concentration that cleans the mold and mildew. A fuzzy or rough appearance indicates the concentration is too strong.
Oxygen bleach may discolor and/or remove old stain and paint from wood surfaces. Always test the cleaning solution in an inconspicuous area.
Oxygen bleach is considered low on the list of hazardous materials, but protective gloves and a breathing mask are recommended when using it.
The use of chlorine bleach to remove mold and mildew is controversial; some siding manufactures like it, while others shy away from it. Chlorine bleach is used cosmetically more often than as a mold-removal technique. You can bleach the color without too much effort. But chlorine bleach also removes color from wood siding and can kill vegetation and cause discoloration. In addition, the mold grows back anyway.
Cover vegetation, and wear protective clothing and eye protection if you choose to use chlorine bleach. Hose down the siding with a garden hose, and use a scrub brush or broom to remove the food supply -- dirt and sugars -- to stop the mold from growing. Allow the surface to dry, and use a garden sprayer to wet the siding with a bleach solution of 3 quarts of water to 1 quart of chlorine bleach. Allow it to remain on the siding for about 30 minutes and rinse it off, always starting from the top and working your way down to prevent streaks.
Do's and Don'ts
Don't be tempted to use a power washer to remove mold. Power washers force water through seams and cracks, causing moisture damage inside walls, and water can penetrate to the interior of the home if a moisture barrier was not installed properly when the house was built. Wash your home once a year with water and dish soap to remove sugars and dirt before mold and mildew begin to grow.
Use stains and sealers with precautions. Water-repellent stains and sealers are typically made with oil mixtures. These oils are consumed as food for mildew and mold. Some contain mildewicide and fungicide to prevent the feast, but the chemicals become ineffective over time, particularity when broken down by ultraviolet rays from the sun. If you prefer stains, use one with synthetic resins instead of natural oils.
Brushes and oxygen bleach are fine for most painted, stained or water-repellent finishes. But don't use steel wool and wire brushes to clean surfaces that have been finished with semitransparent stains or water-repellent preservatives. Iron deposits on the surface react with them, resulting in discoloration.
Paint and Prevention
Paints typically contains mildewcide, with some high-end brands containing more than others. If your brand doesn't contain it, it can typically be added. If mildew is a re-occurring issue, ask your paint supplier if adding more mildewcide to the paint might be beneficial.