Epoxy is in general not very well suited for outdoor conditions. Maybe you can coat it with a weathering resistant barrier coating (with UV absorbing and/or metallic pigments) or metallize it...
Please check following prior art (nano-ZnO and graphene, both at 0.5% w/w) :
Article The influence of nanostructured UV-blockers on mechanical pr...
Article Enhanced protective properties and UV stability of epoxy/gra...
Partially reduced exfoliated graphite oxide should be most fit as it combine protection properties of graphene sheets and strong reinforcement because of presence of carboxyls (the last will react with amine-groups from epoxy-composite. Addition of Fe-salts and subsequent calcination allow to get radioadsorbing ( "stealth" ) coatings.