I used two coats of epoxy mixed with a dark chocolate brown epoxy coloring agent. The coloring was sold to me by RAKA. It took very little - like a teaspoon to a small coffee cup etc. The dark color will do the UV protection work. My friend used black. We wanted them to look like the seal skin boats and they kind-of do.
I'm really happy with it. I'm sure some has penetrated on to the ribs but there's plenty of give and flexability. It was readly available and I had the epoxy etc.
All thes two part urethane coatings are very toxic, very expensive and hard to apply. Usually done with a spray, resperators. They're so toxic, some are only shipped in quart cans. Dangerous stuff! That really turned me off.

Or... you can go the polyurethane in a can varnish type of thing. My other freind uses it with good success. Cheap and works good. Here he usue two to three coats.