Q: I am a novice woodworker and do my finishing with tung oil. I’m currently finishing a table for a friend that will be used in a screened porch in the Southeast. I thought that linseed oil might work better than tung oil here, but I have been told that it may grow mold. Do I have to redo all my work with polyurethane, and if so, can I sand the tung oil and then add poly over it?
A: Either pure boiled linseed oil or pure tung oil will work fine for interior pieces. However, I’d avoid either, and especially linseed oil, on exterior work, since nut oils tend to be natural foods for mildew and can promote mildew growth. You should be aware that while almost all linseed oil sold is either pure raw or boiled oil, many things called tung oil are actually tung oil finish and not pure tung oil. You must read the label carefully. Some tung oil finishes will even work outside.
As for recoating oiled wood, just scrub the surface with mineral spirits on nylon abrasive pads, a process that will both clean and lightly abrade the surface, then recoat with an oil-based finish. For outdoor pieces, use oil-based exterior polyurethane, spar urethane or spar varnish, all of which will go over oiled wood without any problems.