You know, I forgot to follow up on this thread. I finally did get a nice, smooth finish on my bumper using only epoxy. It will fill but it just takes quite a few coats with sanding in between to get the filling effect.
From what I understand, the problem with the surfacing primer on Endura is that it is not as flexible as the epoxy...which is why Epoxy tends to sand poorly.
Here's where we are right now...I'm satisfied with it. Took about 6 coats, 90% probably sanded off....
