MILDEW: CAN IT BE CONTROLLED?
 
 

 Mildew is not caused by stain or paint. According to the The Paint Handbook, "next to water [moisture], mildew is probably the greatest enemy of good paint service. Mildew spores are in the air continuously, and they need only food and moisture to start growing." Mildew is a lower form of plant life, an airborne fungus that can settle on any surface, then grow or colonize on that surface. Among the food sources are wood, wood resins, grass and tree pollens, bacteria, and dirt. The latter three are often found on wood or painted surfaces. According to the National Paint and Coatings Association's Scientific Circular 802, mildew will grow on superficial films of dirt and grease, including industrial pollutants. The National Decorating Products Association warns, "Often mildew that is thought to be growing on paint [stain] is actually growing on a surface film which has accumulated on the paint." It is not uncommon to find mildew on factory-finished aluminum siding or on vinyl siding. Even new lumber when delivered to the jobsite can have mildew spores and developing colonies.

  Mildew spores, which are similar to seeds, are microscopic, and therefore difficult to detect until the spores colonize. The spores are transported easily by air currents, insects and animals. Due to mildew's adaptability and large number of species, it is very hard to control and impossible to totally eliminate. Mildew fungi appear in various colors, but the most common variety is black. For the purpose of differentiation, the algae growth which is often found on wood is green. Algae is a form of plant life that survives on wood that is in a continuous damp state. Mildew and algae are examples of paint and stain disfigurement problems, and they are part of the environment in which we live.

  In order for mildew spores to form colonies that are more visible, they need food, moisture, air (oxygen) and ideal temperatures (usually between 40 and 90F). It is next to impossible to control these four elements -- therefore, paint and stain companies incorporate mildewcides in their products to impede the growth of mildew. Mildewcides are poisonous to these organisms. However, the mildewcides added to paints and stains are not effective in destroying pre-existing mildew.

  Of the four conditions required for mildew colonies to form, moisture is the most important and may be the easiest to control. Plants, shrubbery and mulch in close proximity to a frame house encourage the growth of mildew. Many times, the mildew spores can be transferred from the foliage and damp mulch. The presence of moisture is prolonged because of the foliage. Rain and dew tend to evaporate more slowly. If the siding is allowed to remain in a damp condition over an extended period of time, mildew will spread. Stain and paint manufacturers use the latest, most technologically advanced chemical additives to impede the growth of mildew. But, it is important that the moisture conditions in and around the home be controlled. It also helps if the siding is washed periodically to remove the food sources which contribute to the growth of mildew.

  Mildew can interfere with stain/paint adhesion and cause "mysterious" peeling problems. The Paint Handbook states that if a house is primed in the fall and not given a topcoat until the next spring, mildew can form on the primer during the winter, and when warm weather arrives, the mildew begins to grow. As the mildew grows, it expands, and in doing so, pushes the new topcoat from the subcoat or primer. Topcoats or second coats of stain/paint should be applied as soon as possible after the first coat of primer to avoid contamination by mildew spores. Mildew will attract moisture and promote intercoat peeling. The key factor in the rapid development of mildew is the amount of moisture present in the environment. Lumber at the job site must be protected from moisture for two reasons. One, moisture will promote mildew growth on the raw, unprotected wood before staining or painting. Two, wood that is wet, when nailed in place, will shrink when it dries. Wood should not have more than a 14-15 percent moisture content when stained.

  You can determine the difference between mildew and dirt by applying a 50/50 solution of fresh household bleach and water. If the dark spots turn lighter in color within a very short period, the spots are mildew. If the spots do not change, the contamination is most likely some form of dirt, or black, oily deposits from industrial pollutants. In order to remove mildew, algae, soils and loose wood fibers, Cabot Stains recommends using Problem-Solver Wood Cleaner 8002. Problem-Solver Wood Cleaner is a concentrate which requires adding one part to four parts of water. A mixture of one (1) quart of fresh household bleach and one half (1/2) cup of non-ammoniated detergent added to three (3) quarts of water is also an effective cleaning solution. Either of these solutions can be applied with a garden-type pump sprayer. Wear protective rubber gloves and eye protection when mixing and applying these cleaning solutions.

  Starting at the bottom of the building, spray the cleaning solution on 300-500 square foot sections. Allow the solution to remain wet on the surface for 10-15 minutes, then power wash with clean water. Powerwashing is an effective way of removing the residues of mildew, soils, loose wood fibers and loose paint. Re-examine the surface to insure complete removal of deeply
embedded mildew which may exist. A second cleaning may be necessary, depending on the severity of the mildew, while restaining may or may not be necessary. Please keep in mind that mildew is an airborne fungus and should your home be in a mildew-prone area, the mildew could reappear. It is important to remember that it is beyond the capability of paint and stain
manufacturers to completely eliminate mildew, or guarantee products against mildew growth.

  The Painting and Decorating Association warns, "There are no 100 percent guarantees of success even after the best-known control measures have been taken. Any extended period of warm, humid, and/or rainy weather or tropical storm conditions without much sunshine may cause an outbreak of mildew despite all precautions."
 

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