I molded over a frame that had a primer sprayed on it first. I don't remember the primer exactly but its from PPG and its made specifically for shooting first and doing the bondo over top. The purpose being that you don't get any corrosion between the metal and the plastic filler. I've heard some guys say its unnecessary and other guys who think its super important. Call me in 20 years and I'll tell you if the bodyfiller is still on there good or not.
Can't remember who makes it but there is a filling type primer that goes on extra thick and you can sand it. I used it on my sport bike fairings to fill in some heavy scratches vs using bondo with pretty good results. May want to try that before taking stuff back down to bare metal again.