Spar is always a good bet...I've vaguely heard of poeple spraying it but something in the back of my mind keeps saying that it wasn't the easiest to do. Steve F. uses a spar poly (Helmsman, I think) out of a gun and reports good durability, UV, and ease application. If I ever get that far, I think I will try the auto clear for a couple reasons...I've never shot it and thought it might be a kick, and I've had this kayak site bookmarked for several years that inspires me to try to do it
Toquanatch Creek Kayaks
One thing about spar...it's a proven commodity. And people who know how to use it sure get good results!!!