Protocol Comparison: Laboratory versus Natural Weathering Tests for Performance Evaluation of Coatings on Preservative-Treated Wood

Fourteen stains were tested in the laboratory to compare water uptake and leaching reduction of wood treated with chromated copper arsenate, alkaline copper quat, and copper azole. Based on results of a 2-week test, eight stains were selected to be evaluated over 3 months of accelerated weathering and five stains over 3 years of natural exposure in Toronto, Ontario, Canada. These comparisons were made in order to find a quick and reliable method for replacing natural exposure tests. Comparison of different weathering techniques showed significant correlations between leaching and water uptake results from laboratory tests and natural weathering. The cumulative percentage of inorganic elements leached from coated samples was highly correlated with the cumulative percentage leached during 3 years of natural weathering. Also, the average moisture content of treated-coated samples after 1 and 3 days of water immersion showed a relatively strong positive correlation with the average moisture content of treated-coated wood samples during wet periods of natural weathering (moisture content above 20%, average of 17 reading times). Thus, this quick laboratory test is a reliable short-term test for evaluating the ability of coatings to reduce leaching and water uptake when applied on preservative-treated wood.