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?