Part of the problem is that most epoxy resins are degraded by UV light and either get chalky or change colour.There are also very few UV inhibitors that work well in epoxy. This is why epoxy paints have a tendency to yellow and chalk after a few years.
Unless you are trying, to make a clear coating, you are better off coating the resin with an opaque material to stop UV getting in in the first place. Another technique is to coat green(partially cured) epoxy, with clear poly urethane, this material has to go on when the epoxy is hard but will still take a thumbnail impression. The Urethane has better inhibitors and will protect the epoxy like a paint for a while.
B.E.