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Q.I have often read that wood siding should be backprimed before installation. But I believe that a primer without a finish coat is of no value, because I don’t think a primer can slow down moisture movement. Has there been any research or testing on this question?

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.

Keep in mind that if only the lower portion of the back of wood siding is primed, it is likely that the siding will absorb less moisture, and this would be beneficial. However, it is also possible that if the entire back face of the siding is primed with paint, water that has been absorbed by the siding will be retarded from evaporating. Backpriming with paint retards water vapor from leaving the wood, and the FPL hardboard research suggests that backpriming the entire back with paint may have more negative effects than positive. Unlike paint, a water-repellent preservative will permit drying (loss of water vapor) and also reduce wetting (liquid water).

If I were installing new solid wood siding, I would treat the entire board (front, back, ends, and edges) with a paintable water-repellent preservative and allow two days for proper drying. Best of all, dip the siding before installation (redipping any cut ends), then dry, prime, and topcoat. If that is not possible, I would treat the back, the ends, and the bottom edge of the siding with water-repellent preservative.

Finally, if a paintable water-repellent preservative is not available, I would backprime the lower half, ends, and bottom edge of the board with a single coat of alkyd primer paint.