[Gippetto]

"Epoxy goes on bare steel to prevent corrosion.
Filler goes on bare steel too so spray the entire panel, then sand it off just before applying filler.
This is the proper process"

Question to Footstomper: When you say sand it off do you mean break the surface to give it some tooth for subsequent coatings, either paint or fillers? If you sand the epoxy off you destroy the moisture protection provided by the sealer. Filler absorbs moisture.