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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.