IMO(and others as well)!!! If you are going to use epoxy as your primary bare metal primer then it should be a continuous cover for best moisture protection then the filler applied. When you are finished with the filler prep to your liking, then you re-epoxy to sandwich the filler between epoxy coverings and cover any bare spots from sanding. Next on is the 2K.

BUT, with those good words said, many still do the filler first then epoxy and are sucessful in doing it that way. Which is the right way - I do it as I said first.