- Q.I'm doing an addition on a
house I built several years ago. Some of the eastern
white pine exterior trim — which was primed on
all four sides with oil primer and top-coated with
white latex — is showing sap stains. None of
the knots or obvious sapwood was preprimed with BIN
(www.zinsser.com), our usual
shellac-based stain-blocking primer. Would it do any
good now to go back and recoat the dark areas with BIN
before recoating with latex? Or is it too
late?
A.Jon Tobey, a painting
contractor in Monroe, Wash., responds: Yes,
you can easily reprime these areas, with a few
caveats. First, don't use water-based primers for
tannin stains, because these stains are
water-activated. Second, because alkyd and other
solvent-based stain-blocking primers are very
brittle — and therefore fail easily under
expansion and contraction caused by temperature
changes — you'll need to apply a thin,
flexible coat. On larger jobs, we use a sprayer to
do this, fogging knots and stains with a very light
coat of XIM X-Seal (440/871-4737,
www.ximbonder.com), PrepRite
ProBlock(800/474-3794,
www.sherwin-williams.com), or a
similar alkyd-based stain-blocking primer. On
smaller jobs, rattle cans are an easy and
cost-effective option. I don't recommend using
brushes or rollers; they leave too thick a coat,
and these primers dry so fast the tools would be
ruined.
Finally, while it may look as if only the
knotholes are the problem, the entire board may be
bleeding, but to a lesser extent; this may become
apparent once you fix the knotholes. Something
similar happened to us once: We spot-primed all the
knotholes, then were called back two years later
because the rest of the house had yellowed (it was
covered with big light spots). After that, we began
lightly fogging entire houses with unthinned
alkyd-based stain-blockers (at a rate of about
1,000 square feet per gallon). We haven't had a
callback since.