Measuring the Effectiveness of Water-Repellent Preservatives

Four years of outdoor exposure tests and laboratory tests were conducted at the Forest Products Laboratory to determine which laboratory methods are the most accurate in measuring the effectiveness of water-repellent preservatives. Eleven solutions–one preservative, one water repellent, and nine water-repellent preservatives–with a wide range of water repellency–were tested on kiln-dried Ponderosa pine. Seven laboratory tests–swellograph, flowing water, swellometer, water spray, cupping, weight gain at 30 to 80 percent relative humidity, and weight gain at 65 to 97 percent relative humidity-and two weathering tests–right-angle specimen and window sash–were conducted. For the laboratory tests the specimens were dipped for 30 seconds in the solution, and for the weathering tests the specimens were dipped for 3 minutes. The degree of agreement between the laboratory and outdoor tests was expressed as a correlation coefficient, with 1 representing perfect agreement. When compared with the window-sash test, the swellograph test had a coefficient of 0.9222, and the test for weight gain at 65 to 97 percent relative humidity had a coefficient of 0.814; the latter number was considered significantly lower for the number of solutions compared. The coefficient for the other laboratory tests–water spray, cupping, and weight gain at 30 to 80 percent relative humidity–ranged from 0.872 to 0.939 and were not considered significantly better or worse than the coefficient for the swellograph test, which is the present NWMA standard test. A 3-minute dip of window sash in a good water-repellent preservative before painting will materially reduce or eliminate stain and decay fungi, end-checking, cracking of the glazing compound, excessive swelling, and paint failure. Treatment with a good water-repellent or water-repellent preservative provides better protection than treatment with a preservative alone.

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