Molded kayaks are usually made of polyethylene, an otherwise extremely durable material which can be damaged by the sun. The used market is full of kayaks manufactured ten or twenty years ago which spent some part of their life outdoors. There are many variables in manufacturing (UV stabilizers, pigments, even simple thickness of the plastic), maintenance (application of UV protectants) and storage (occasionally on the roof of a car vs stored outdoors for years on end).
Given the above, how can one tell if the weakening/embrittlement of the plastic is enough to significantly reduce the durability of the kayak?
I am not worried about dents or “oil canning” which may be a result of a short time in the sun, as much as structural integrity of the hull (retaining the toughness and reliability that is the great virtue of molded plastic boats).