Can I run finished wood through my planer?

Q:

Will planing wood already finished with polyurethane be hard on carbide planer knives? I’ve heard the finish bakes onto the knives, eventually causing them to overheat.
—Sander White, Barrie, Ont.

A:

You could plane off an old finish, but it’s not worth the risk. Planer cutterheads can generate sufficient friction to soften such finishes as polyurethane, gumming up their knives. Instead, use a belt sander set to about half its maximum speed and an 80- or 100-grit belt to remove the old film finish, as shown above. Even if the belt gums up, that’s easier and cheaper to replace than planer knives. After sanding away the finish, feed the wood through a planer to erase the sanding marks.

If you’re recycling old boards or ones from an unknown source, take added precautions. Treat any old painted finishes as though they contain lead, and remove them with a chemical stripper. Wear an appropriate respirator, work in a well-ventilated space, and protect exposed skin. Before feeding recycled lumber through a planer or jointer, use a metal detector to check each board for hidden nails, screws, and staples. 

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