Al Breed replies: Use Epifanes marine varnish, gloss, with UV blocker. If the door is facing the sun, you will still have to redo it every once in a while. Ask a boat person for details.
Jeffrey Cooper replies: Epifanes, available from West Marine—westmarine.com.
Garrett Hack replies: Marine varnish—high quality spar varnish.
Bob Couch replies: There is pretty much no exterior natural wood finish that doesn’t require annual or bi-annual maintenance. Fine Woodworking did a two year study of all available exterior finishes on different types of wood. As I recall, this test was done in three different areas of the country—Torture Test for Outdoor Finishes. Epifanes was named the best and it had the highest UV filter. It is a marine finish made in Holland and imported by Epifanes North America in Thomaston, ME—epifanes.com.
The instructions are to first use a coat of penetrating epoxy to really seal the pours of the wood. I get my penetrating epoxy from Jamestown Distributors—jamestowndistributors.com. Epifanes even come in half quarts which is what I usually buy.
The exposure any door has to the weather, especially the sun, can have a big effect because of UV, which is your biggest enemy.
Elliot Savitzky replies: Most spar varnishes are a mixture of hardening oil (linseed or tung) and alkyd, phenolic or polyurethane resin in a solvent. They have enough natural pigment to block ultraviolet sunlight. The surface you are painting must be clean and dry, but once the varnish cures, it protects even underwater.