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Polyurethanes, traditional varnish and, for that matter, any clear finish will get more brittle with age. The reason old, flaking varnish curls outwards is that the outer surface becomes shorter than the inner surface due to the extra surface cross-linking from the ultraviolet light.
There are special chemicals designed to trap and neutralize these free radicals before they can do their damage. They are called antioxidants (something like vitamin-E, actually) and they work the same way your antioxidant vitamins work to keep you healthy.
Interestingly enough, conventional varnishes cure by a chemical reaction between the oil and the oxygen in the air. This is called oxidation, and the addition of antioxidants to a conventional varnish would poison the curing reaction. It is therefore impossible to add antioxidants to varnish and thus any varnish will lose its flexibility fairly rapidly with exposure to the sun. Some chemically cured coatings such as two-component polyurethanes are compatible with both ultraviolet absorbers and antioxidants, and those have the best maintenance of gloss and flexibility. Versions which are VOC-compliant are available for high-end applications. My company has been making such coatings for over fifteen years.
What about the free radicals,,, damm those bastadges .. 2 part component polyurethanes.. Hows about moisture cure ? floor products ,, my bud says his finish on his kitchens wood floor is 20 yrs. old ..looks pissa .. I guess what I'm saying is all this worrying about the rocks,,the water,,the salt .. Your plugs worst enemy is the Sun ... UVL just breaking it all down ..
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