I have had that exact situation happen in one of my classes. Student basically finished his kayak and when we took it outside to wet sand for varnish, the soft epoxy under the hardened very quickly turned whitish and softened in the hot sun. Without knowing how far the damage went, he stripped it all off and re-glassed, rather than have the area expand by doing small patches.

I also was once brought a canoe that hung in a barn for over ten years. The wife wanted me to finish it for her husband for their tenth anniversary (he began the canoe before marriage and then kids). Once I began work on it I found soft spots and began to pull off glass/resin that had not hardened in over ten years. I did just patch as much as I could determine was soft, but hard to know for sure if all was removed.