you have to watch your barrier coat with epoxy as with like say ppg dp it will roll off on feather edge and if not when you primer the panel it can lift edge up .filler like Z grip and duraglass i put right on the substrate and you get better hook up and no roll up. the glass fillers can be finsh off as for pin holes the heavy long strand glass fillers will have voids in it but the mar glass or durglass i finshed off many times . bondo is a name of a body filler but more a name for a half ass job like bondo bucket