I know I can prime and paint medium density fiberboard, but is there any other type of finish I can use?
Kevin Hancock: Shellac will seal MDF, though it takes a few coats. I’d suggest de-waxed shellac. Zinsser SealCoat is a good de-waxed shellac.
Michael Dresdner: Actually, any common finish will work over MDF. The problem is that it takes more coats than normal, since the MDF absorbs a lot of finish, and may swell with waterbased coatings. While solvent- and oil-based finishes, like shellac, lacquer and oil varnish, will work, you still end up with something that looks like MDF under the clear coating, which may be the biggest drawback of all.