 Information About Exterior Mildew the fungus among us Mildew is a chronic problem in the Pacific Northwest. This information advises on the relative mildew resistance of coatings systems and procedures for mildew control. Mildew is the reproductive offspring of fungi. Fungi are plant-like parasites. Fungi take nourishment from living and dead plants and other organic matter. Over time the host being attacked decomposes. Fungi are one of the most prolific life forms in nature. They are in the soil, on vegetation, and on exterior structural surfaces. Dormant mildew spores become airborne and are carried with the wind to new hosts. Fungal growth requires the right balance of oxygen, water, and temperature within the nutrient source. A living tree is mildew-resistant. Its hollow wood cells are full of water. There is not enough room left in the cells for the oxygen required to support fungi. Water evaporates from the cells of cut lumber. The wood moisture content equalizes with the humidity and fluctuates with changes in humidity. Air replaces the evaporated water in the cells, furnishing oxygen for fungal growth. Fungal growth occurs when the temperature is between 40 and 60 degrees and the wood moisture content is 15 to 35%. Preventing fungal growth in the wood requires sealing it from outside moisture entry sufficiently enough to keep the wood moisture content safely below 15%. Fungi do not require sunshine to survive and reproduce. Sunshine does influence fungal growth. Optimum conditions for fungal growth are not normally present in winter months. Fungi go through a dormant period. Sunshine in the spring will raise temperatures, reactivating fungal growth. As summer approaches the sunshine dries the wood and temperatures rise until the temperature is too high and the wood too dry to support fungal growth. Fall weather changes reactivate growth again. In the Pacific Northwest optimum mildew growth conditions are present six months of the year. Mildew is more than an aesthetic problem. It causes wood decay. Mildew introduces enzymes into the wood that in turn allow for bacterial growth. Bacteria accelerate wood decay even further. What can be done to prevent mildew? While total elimination is almost impossible mildew can be controlled and minimized with the following preventive measures. Don't assume there is no mildew. The early stages of mildew are not visible. If you coat over mildew it grows underneath the coating. You then have to fully remove the coating to eliminate the mildew. That's a lot of work. New bare wood, aged bare wood and previously coated wood are all susceptible to mildew. Sap stain and blue stain are forms of mildew on new wood. Assume there is mildew present before coating the wood. Remove the mildew with a mixture of one quart household bleach, a half-cup of laundry detergent, and three quarts warm water. Heavier mildew deposits may require some scrubbing. Thoroughly rinse the wood and allow it to dry after treatment. If you use a commercial cleaner, make sure it contains chlorine. Chlorine is the active ingredient that kills mildew. Take adequate precautions to avoid skin and eye contact. Use a coating with mildewcide in it. The coating contains organic matter. It is a nutrient source for mildew. Mildewcides add toxicity to the coating and inhibit mildew growth. Mildewcides lose efficiency over time but are very effective in the first 3 to 5 years after application. Additional mildewcide can be purchased at most paint stores and added to the coating of your choice for increased mildew inhibition. Coatings selection is heavily influenced by aesthetic preferences and performance is sometimes sacrificed to achieve aesthetics. Poor mildew resistance in a coating is a performance weakness that quickly becomes an aesthetic issue. With some exceptions coatings mildew resistance can generally be rated by coatings type. Clear finishes and stains have less mildew resistance than fully pigmented paints. Oil base products have less mildew resistance than latex. Flat paints have less mildew resistance than eggshell and satin paints. The addition of mildewcide will improve the mildew resistance of all these coatings. Assuming equal levels of mildewcide are added to all of them, their mildew resistance rating in relation to each other will not change. Maximum mildew resistance on bare wood is provided with a three-coat paint system consisting of a primer coat of penetrating oil primer and two finish coats of a high quality acrylic latex. Both primer and finish should contain adequate mildewcide. Multiple coats improve the mildew resistance of stains and clears. Multiple coats seal the wood further and lessen the amount of moisture entry into the wood. Two coats of semi-transparent stain are recommended. Depending on solids level, clears may require 4 to 6 coats to adequately seal the wood. Brushing is recommended over rolling or spraying. Brushing forces the coating to penetrate more deeply into the wood cells. This provides deeper sealing against mildew- inducing water penetration. If the wood has not been installed, additional protection is gained by prefinishing. Prefinishing allows coating of all sides and edges for a total seal. If the wood is already installed, make sure all joints are caulked and sealed and all exposed edges are sealed. Regular maintenance will extend the life of all coatings and control future mildew growth. Maintenance includes the cleaning procedure described earlier. Exposure conditions determine how frequently you should clean. Clears and stains may need annual cleaning. Paints normally need cleaned every two to five years. It's best to clean at the end of spring. Maintenance also includes repairing any defects in the coating, like small cracks and peeling, as soon as they are discovered. It's a good idea to thoroughly inspect all surfaces for any defects after you have performed the cleaning. Repair any defects you find. Lastly, mildew can be controlled further with prudent gardening practices. Plants, trees and shrubs should not be allowed to grow within a foot or two of the house. Keep dirt away from the wood. Avoid watering the house when watering plants and shrubs. This will lessen the potential for mildew growth on the house. Properly maintained clears have an average exterior life expectancy of two to four years. Stains will last three to six years. A three-coat paint system will last ten to 15 years. Select a coating to suit your taste and need. Make sure it has mildewcide in it. Clean the surface before application. Brush-apply the coating. Put on enough to seal the wood. Maintain it properly. Practice good gardening habits. These procedures will minimize mildew growth, prevent wood decay and prolong the life of your coating system. |
