Ok I am going to put my two cents worth in on this. I have been an architectural and industrial coatings rep for 15 years and here is the rule of thumb when painting or preping bare metal surfaces. Metal is porus in nature. Most fillers do not actually penetrate that deep in the substrate wheras an epoxy primer both penetrates and seal a barrier coating depending on the type of epoxy and what solvent base it uses. There are many variations of types of epoxy primers and everything from polysiloxane epoxy primers to urethane based xylene and tolulene based primers. All of which have far better adhesion properties than any body filler ever thought of. Once a metal surace has been sealed and locked in then the primer can be roughed up to make the surface tolerable for the body filler to adhere to. The primer coat acts more of a bonding agent and sealer for the topcoat than a topcoat.
Hope that helps.