Epoxy itself usually only needs to be topcoated to prevent chalking due to the sun. If I were going all-out on the restoration, I'd paint the inside of the fenders with epoxy and topcoat with some cheap black single stage. Undercoating is probably just extra weight in this application (that is my opinion, no facts to back it up). Look into the Omni line of epoxy and MAE acrylic enamel for this (and your underbody, floor, suspension parts, etc)- decent stuff and low cost.
That said... I brushpainted two coats of Rustoleum on the inside of my fenders, on top of the factory coating. So far so good.