Spar Urethane may yellow over time, but it's not noticeable the way I do my finish...any crown paint goes over the spar urethane then under my topcoat of gasket lacquer. This type of finish is more work, but it pays off IMO. Spar Urethane is as waterproof as you're gonna get for a finish, and the gasket lacquer really adheres to it outstandingly and creates an EXTREMELY tough finish. I've shot arrows with this finish extensively into hay bales, 3D Targets, bag targets with nothing sticking to it and no noticeable wear on the finish. My prototype testers came up with the same result. It's pretty heavy, but there's a lot of flexibility to how much weight you add depending on how many coats you use of each finish.
I do 2-5 (I can get away with 1 coat if it's the right consistency, and if I'm kind trying to keep the weight as low as possible) coats of spar urethane thinned with Penetrol (this stuff is great, I thin my crown paints with it as well and something about it really makes for a glass smooth finish) with steel wool or scotch-brite between coats, then move to the gasket lacquer with 2-5 coats as well.
Craig