M_Boats: varnish

Hate to get off the interesting thread about teeny motors, but I have another question; what resists UV best on a boat in the sun: standard spar varnish, or urethane spar varnish? Additionallly, I now have numerous coats of regular varnish in good shape, and I wonder if the urethane would go over the regular without sanding to bare wood (which I ain't doing). Thanks, Tom Jenkins