The Role of Water Repellents and Chemicals in Controlling Mildew on Wood Exposed Outdoors

Book excerpt: The natural look of wood siding, and the retention of that look, has become increasingly popular over the past several years. Many new commercial formulations are being offered for use as clear natural wood finishes. A successful finish will retain color, control mold and mildew, and minimize weathering. Some of these formulations contain a chemical as a mildewcide (fungicide, preservative) and/or a water repellent, and some do not. To evaluate several chemicals (alone or in combination with water repellents) as components of natural finish formulations, we dip-treated ponderosa pine sapwood specimens in various formulations and exposed them on test fences in Mississippi, Wisconsin and Washington. Several chemicals were very effective at controlling mildew and maintaining the natural appearance of the exposed wood. Aqueous formulations of chromium salts were the most effective for up to 22 months exposure outdoors. Many chemical formulations in mineral spirits (including water repellents and wood sealers) were effective for 5 to 9 months of exposure, but their effectiveness was reduced with longer exposure. Of all the chemicals evaluated in mineral spirits formulations, only pentachlorophenol and copper naphthenate were found to be effective mildewcides after exposures of greater than 9 months. Their mildewcidal effectiveness was improved with the addition of a simple water repellent (paraffin wax), especially with exposures greater than 9 months. Mildew growth was markedly worse in the warm, humid climate of Mississippi than in Washington or Wisconsin. This means that natural wood finishes for climates similar to Mississippi must contain high concentrations of effective mildewcides for good performance. This paper should be useful to homeowners, architects, builders, and wood finish formulators.