A.Bill Feist, a former wood-finishes researcher with the Forest Products Laboratory in Madison, Wis., responds: Normally, I encourage people to use boat paints and varnishes for a number of nonmarine uses because of the general high quality of these finishes. But in this case, I think doing so might be a mistake.

Even though polyurethane enamels are made for many different surfaces on boats, polyurethanes in general are known for their brittleness — and enamels are even more brittle than flat or satin paints. This brittleness is okay where there is not a lot of wood movement. But an older wood window — especially the sill — is subject to considerable...

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