I just finished glassing a small Corsair that I scratch built. I used Great Planes resin which I understand is similar to Zpoxy and shot it with Rustoleum primer. I was told by a gentlemen in the warbird area that after the resin and before the primer, lay down some Spot Putty over the entire airframe. I thinned it with acetone and it covers beautifully. Even better is that it sands better than primer and it is wet sandable too. After you can put on a thinner coat of primer and wet sand out. I can't tell you how much better the finish is on the top wing(where I used the Spot Putty) than on the bottom (only primer) It is a hidden secret better than penicillin.

It is made by Bondo and comes in an orange tube, SVX