You're right, should probably upgrade your finishing materials. If you don't have spray equipment you might consider paying to have someone spray an automotive clear coat on your kayak. Your car's finish doesn't deteriorate like that, right? There are also commercial marine coatings like AwlGrip, but they need to be sprayed as well. Betsie Bay kayaks are sprayed with AwlGrip 2000, and they last very very well.

If you don't want to spray, why don't you call Epifanes or Interlux and see what they'd recommend?

m