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
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.