A.Wood finishes expert Bill Feist responds: Some architects and wood trade associations advise priming the back side of solid wood siding with paint or a water repellent. Research performed at the U.S. Forest Products Laboratory (FPL) in Madison, Wisc., during the 1950s conclusively indicated that backpriming the siding with a paintable water-repellent preservative (like DAP’s Woodlife) improved the paint retention and overall performance of horizontal wood lap siding. However, the benefits of backpriming solid wood siding with paint have not been experimentally verified, and I am not aware of any published information on studies on this subject.
Recent FPL publications describing studies on backpriming hardboard siding conclude that "backpriming did not improve the performance of this particular hardboard siding nor did it lower in-service moisture content." The studies indicated that there was a possibility that backpriming the lower half of the hardboard siding was beneficial.
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