I'm using epoxy primer. The body work goes on top of the epoxy. If I wasn't using epoxy, I would do the body work right onto the metal. Somewhere in this thread I posted a video of an epoxy/rage gold torture test. And let's just say, the epoxy stuck to the metal insanely good, and the filler stuck to the epoxy insanely good.
I'm only doing epoxy instead of ryoken green (aka zinc chromate) do it's popularity and how common it is in resto work. I firmly believe that the ryoken green will stick and prevent corrosion just as well, and likely better than the epoxy, but due to the popularity of the epoxy, I just chose to go that direction.