An instructor in my neck of the woods did a class effort Kayak to be given away for charity a few years back and utilized a nearby car dealership to put a Urethane finish on the craft. It looked fantastic although I not sure how it survived everyday use along with expansion and contraction movement under the sun and changing temperatures. The price of automotive paint is getting pretty expensive, not sure if I'd chance it. Perhaps you could do some test strips with various coatings over the winter with plenty of exposure to determine durability.
Mac