I'm nowhere close to being an expert on paint and bodywork, but I now know far more than I ever wanted to about the craft.
Wrenchnride has the right idea. When you apply your filler directly to bare metal you will get the best adhesion. On my car, I rubbed the spots to be filled real good with 80 grit to give the filler a good hook into the surface.
For primer, you can use Urethane based 2K primer, or Epoxy primer. 2K primer is good high build material. Most can be applied to bare metal, builds up good to give you some sanding room, and dries flat with no orange peel to sand out. Epoxy primer can seal out moisture, most will apply to bare metal, and can be bondoed over if needed. A friend in the biz recomended
http://www.southernpolyurethanes.com/ for their epoxy primer. He said it is user-friendly for DIY projects, and high quality for professionals. They're semi-local too, so you'll be supporting Georgia's private commerce sector
.
If your car's got a lot of small low spots to fill, or metal/filler spots to be blended you can use Dave's favorite primer, Evercoat Slicksand which will build
extremely high, and dry hard. I found out it's often used to repair hail damage, and door dings. I used it on about 80% of mine to make sure I filled all the small creases and dings, and to blend the welded repairs I did. It works great in place of blending putty, but dried with an orange peel that needed some dry sanding.
Just don't use a self etching primer. Anything polyester based (most fillers, some surface primers) won't stick to it. The stuff I used dried very slow for some reason, and was a pain to remove from the areas that needed filler. Actually, by removing it from places that needed filler really works against the purpose for using it in the first place, which is to seal the bare metal.