UV exposure will eventually break down epoxy, and will certainly accelerate the aging process of your board. Generally the paint will be first to go, and if redone before it becomes porous, the epoxy undrneath will be fine. Heat itself is another issue, as when epoxy gets hot, it gets soft. This can cause a condition called print thru, which means that the weave becomes visible thru the finish coat.
Short answer... Its undesirable, but on a boat often unavoidable, to leave the boards in the sun...