When you think about the strength of epoxy, it’s almost hard to believe that the sun can damage an epoxy resin finish. As with many yellowing issues, the culprit is the sun. Specifically, yellowing of epoxy is caused by UV rays which break down the fibers in the epoxy and cause the whole thing to take on a dull yellow hue.
Thankfully, many manufacturers have added UV stabilizers to their epoxy resins. UV stabilizers will prevent yellowing, but not forever. In the end, a UV stabilizer just delays the inevitable. However, there is another additive called HALS (Hindered Amine Light Stabilizer) which is far more effective over the long haul. This stuff prevents yellowing before it starts and has performed very well in tests and experiments.