Spar varish. Coat the whole thing one good layer (2 or 3 thin layers) then sprayed the seams heavily and tilled the box making it run around the seams. Stuff lasts forever (several years) on outdoor stuff and has not become a problem after a few months.
Started 9/13, building slowly, not trying the no treatment anymore
Heat mineral oil and bees wax or parafin in a double boiler until all the wax melts, Pour it in a bottle and let it cool. Coat the wood with it and it will be waterproof. Coat the feeder with it and it will seal the joints.
If anyone is still following this post I chose to use the fiberglass resin. It was easy to use and dried hard and sealed well, with no leaks. The smell of the resin during the application can be a bit overwhelming if you don't have proper ventilation. Once cured properly ther was not any noticible odor. I feel it was a good choice.
image.jpgAll this sounds like a lot of work. Look up fatbeeman feeder with a no leak milk jug. It uses a mesh ladder and a chute for bee access and can easily hold a gal or more. This design is very adaptable in size. Nuc, 10frme,8,whatever. No need for resins and polyesters and all that. Just use a plastic jug. I use a gal ice cream container. Seem to have a lot of those around my house.