May 20, 2021 - 01:01 PM
Generally speaking, epoxy tends to yellow or chalk when exposed to direct sun light for extended periods of time, which is why we recommend protecting epoxies, MAS being no exception, with a UV protectant varnish or paint for exterior applications. All epoxy resin systems are not created equal. There are some UV resistant epoxy laminating systems out there that are slower to yellow over time, but I don’t know of a system that is 100% UV resistant.
Marine grade polyurethane varnishes, with UV protection additives, offer improved UV stability and abrasion resistance over "off the shelf" spar varnishes available at hardware stores like Home Depot or Lowes. We manufacture Bristol Finishes, which is varnish ideally suited for overcoating epoxies, but Epifanes, Alexseal and Interlux are a few brands that our customers have been happy with. Many different varnishes are available; single component, 2-part, high gloss, satin, clear or tinted with color. Automotive clear coat also works well.
When varnishing, first and foremost follow the varnish manufactures instructions for overcoating epoxy. If no instructions are available, then simply treat the varnish as an additional coat of epoxy: Allow last coat of epoxy to cure 24 hrs, scuff sand with 120-220 grit paper, clean sanding residue from surface, and apply varnish.