As well we should not forget that heat causes condensation when applied over bare metal will cause rust which we see a lot in old lead jobs
, but this is (according to I CAR) the reason for the epoxy primers under modern fillers, not so much for adhesion but for controlling rust. Modern fillers while under going their chemical change heat up and yes can form condensation on bare metal. I also was uneasy applying filler over cured epoxy even though the paint companies tell you it is o.k. I still prep my panel or grind like I normally would, apply epoxy,: once it has cured then hardblock the area with 80 grit before applying the filler
