Epoxy Primer. You can use filler over it (after roughing it) and If you have to hammer out a dent, you can always spray more. It is by far the best way to go. If it will sit around for awhile before top coating, you will need to sand it for good adhesion.
If you plan on using POR-15, apply it directly to the metal, not on top of the epoxy. I did this (a recommendation from a local paint shop) and where I did not sand the epoxy enough, the POR-15 did not adhere very well. I won't be taking advice from that paint shop again!
