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).

  • FYI, I have a plastic kayak 35+ years old -- still in "perfect" shape, but it never spent much time in the sun or strapped to a rack. – Martin F Mar 14 '19 at 19:54

As long as it still flexes you're good to go.

Part of the strength of polyethylene kayaks is in their flexibility, apart from colour loss, the most significant aspect of sun damage to the plastics is that they become brittle. If pressing on the larger flatter sections causes it to flex elastically then the sun damage is not yet significant.

This is more of a risk for older boats, the plastics used have changed over the years and modern boats are less susceptible.

You should also check any fittings for sun damage, for example the softer rubber of hatch covers is more vulnerable due to its greater need to flex and stretch.

Oil canning is usually a result of bad storage or transportation rather than sun damage, the easiest way to get it is overtightening the straps on roofbars. One of the solutions is to leave it out in the sun for a while.

  • At a risk of going off at a tangent, does the fix for oilcanning actually require sunlight, or just warmth? – Toby Speight Mar 11 '19 at 14:51
  • @TobySpeight. just warmth, but sunlight is safer than a heat gun. I think we have a question about it already – Separatrix Mar 11 '19 at 14:57
  • Definitely only warmth rather than heat. I was just asking since the question is about sunlight damage (which obviously takes a much longer exposure). It seems (superficially) like a contradiction to recommend extra sunlight! – Toby Speight Mar 11 '19 at 14:59
  • @TobySpeight, you're actually trying to heat it enough to soften the plastic and trigger "material memory", though you may have to apply gentle forces as well. It's possible with a heat gun but there's risk of serious damage. Sunlight damage is a matter of years of exposure, fixing oil canning you're talking about a couple of hours. – Separatrix Mar 11 '19 at 15:25
  • 1
    Found the existing question: Restoring a deformed kayak hull – Toby Speight Mar 11 '19 at 15:29

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