Hi Jan, looks like you are getting closer to building something fun. My only thoughts about epoxy/graphite coating is that is is used on the bottom only. The stuff soaks up heat from the sun incredibly. When I used it just on the bottom center strip of my boat and had the boat outside upside down in the sun (air temp 95) the graphite strip was too hot to touch while the varnished hull was simply warm in the sun. The very hot strip was a bit soft, then hardening up in the shade. I can't think that in the long run making a solar oven out of a kayak is a good idea. So while graphite may affort some UV protection I think it is mainly useful for making a slick surface. If this boat is a prototype why not hold off painting it untill you've paddled it awhile and get some artistic inspiration for prototype decortations?