I have used thousands of gallons of body filler, and never put epoxy under it. Just be sure the metal is clean.
Knock on wood. I've never had rust problems under filler, or had any come off.
If you don't make mistakes. your not doing anything.
69 ss rs full custom camaro 98 ISCA grandchampion
69 ss rs bb camaro wifes driver
66 Elcamino 350/all dz parts,ac,windows,loaded,my driver
69 ss chevelle bb conv.fresh frame off
26 T sedan street rod