Last night I read (The New Complete Guide to Beekeeping by Roger A. Morse, 1994, p. 113) that "We use carefully planed wood to make frames and hive interiors to reduce the amount of propolis bees can collect." Question: Will coating frames & hives interiors with several slick coats of varnish be worth the time and effort?
If you planed it that should be good. I would not put anything on inside of the hive. With wood you need to have a certain level of breathing for the wood. You should be fine if you have a smooth planed surface.
If you look at it from the bee's point of view, their natural habitat is wood. Not varnish. Varnish will not be what they naturally want to live in, won't smell like wood, or feel like wood. It seems like the tolorate plastic, but they don't really like it. I think it would be a bad idea to varnish or paint the interior.
I admit, I've never tried it, because I thought it was a bad idea.