by Marie Tupot Stock
Although wood siding has lost market share to vinyl and
fiber-cement in recent years, sales of wood siding remain
strong in many parts of the country, especially in the northern
half of the U.S. Increasingly, when wood siding is chosen, it's
being installed preprimed or prefinished. "It's becoming rare
to see a builder install raw wood on the exterior of a house,"
notes Tom Daniels at Cabot Stains.
Machine priming of siding was first developed in the 1940s,
but the use of preprimed siding didn't take off until recently.
Fred Churchill, owner of a machine finishing company called
Churchill Coatings in Grafton, Mass., recalls, "When we started
more than 30 years ago, the lumberyards laughed at us." Today,
a wide selection of wood siding, as well as some fiber-cement
and composite siding, is readily available preprimed or
Bracing For the Elements
Although wood siding has been installed unpainted for hundreds
of years, there are several disadvantages to doing it the
old-fashioned way. The unpainted siding can pick up moisture
while being transported or stored on site, meaning paint is
less likely to adhere. Moreover, when unpainted siding is
exposed to the sun, it loses its paint-holding ability and can
begin to cup. "We used to buy hemlock, fir, or spruce. Within
two days in the sun, it warped," says Jesse Head of Head
Construction in Middletown, Calif. "It's not worth bringing on
the job." Head, who builds two or three homes a year, hasn't
put up a bare piece of siding in over five years.
According to Mark Knaebe, wood surface chemist at the USDA
Forest Products Laboratory in Madison, Wisconsin, sunlight
destroys the lignin near the surface of unpainted wood. Lignin
is the glue that holds the cellulose fibers together. "When the
sun destroys the lignin, the fibers are still sitting there,
but they are not glued very well to the wood anymore," says
Knaebe. "If you paint the siding after a couple of weeks of
exposure, you're painting over those loose fibers." In
controlled tests, wood exposed to the sun for four weeks before
painting failed five years earlier than wood painted
immediately. Most experts recommend that wood siding be primed
within two weeks of installation.
Factory-primed siding is produced by passing the siding through
a machine coater, which floods the wood with paint. As the
board leaves the machine, it passes through a roller or set of
brushes designed to remove excess paint (see Figure 1).
1. As a piece of siding is pulled through a
machine coater, it is flooded with paint (top). On the
other side of the machine, rollers or brushes remove
the excess paint (bottom).
Some prefinishers use a vacuum coating machine, which sucks
the excess primer off of a board that has been flooded with