Plastic kayaks from reputable brands come with UV stabilizers in the plastic, such that a boat would have to have years of cumulative sun time before it shows sun damage.
If a boat is stored outside in sun, the signs of sun damage you would see would be some fading of colors (and brighter color plastic where sun doesn’t hit, like under straps or inside hatches). This is cosmetic and is not a failure of the boat. What the sun does do that could lead to a failure is that it:
- makes the plastic more brittle
- makes the plastic less likely to accept plastic welding as a repair.
So the failure would be a hard hit causing the plastic to crack, and the plastic not being repairable. Even the most sun damaged kayak could still work fine, so long as it doesn’t get that hard hit. At the places I guide.teach for, we have lots of Necky kayaks from the 90s which are still going strong, and all live outside with limited sun protection.
But a brand new, non-sun damaged kayak could also be broken with a hard enough hit. Theoretically should take a harder hit than a sun-exposed one, and would be more likely to be repairable with plastic welding than a sun exposed one, but it can still happen. The one kayak I broke was about a year old one that was stored inside which I cracked by the skeg box due to going over a pour over without enough water, and the stern slammed down on the rock (that same pour over has stripped a rudder off another boat I had due to the same thing - too bad it is such a fun pourver, as it is also a costly one). That cracked skeg box boat I sold cheap to a friend who is more handy than I and is still going strong.