Neal & rf, spar varnish, or marine varnish, is absolutly best for doors. With high oil to resin ratio film is flexible so door swelling and shrinking don't crack finish.
I prefer "real" varnish over polyurethane. That would be one with phenolic resins rather than poly. I prefer alkyd over poly. Most now have combinations. Tung oil based is tougher than linseed, again most have combinations.
Ideal would be a high oil to resin ratio, more tung oil than linseed or other, more phenolic than poly resin, and UV inhibiting additives.
Check can for recommended thinner and ratio for using applicator instead of brush. Using brush can be tricky, poly can be even trickier, you don't apply it like paint, where you basically apply paint then brush it out. You want varnish to flow off wet brush in one stroke. With applicator (folded piece of T-shirt) you thin varnish considerably, ( sometimes 50-50) moistenpad in thinner, dip into varnish, and make long slow wipe. Overlap about half on next stroke. Takes longer, more coats, at least twice as many, but no brush marks and thenner varnish pentrates deeper.
Don't forget top and bottom of door, and inside hinge mortises and handle and latch holes.
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[CENTER][FONT=Comic Sans MS]Look at the nail, not the hammer. Watch the fence, not the blade.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]