Conversion Varnish works well. Watch out though, one of the chemicals, (I cant spell it) will literally kill you so wear a good respirator if you spray conversion varnish.
(As a note, conversion varnish for use in commercial applications is reason for the EPA to climb all over you in Ohio. I dont know about other states.)
My dad is a cabinet maker and we have some out in the shop, but it's only for cabinets.