Etching primers were designed for increased adhesion back when lacquer primer was the only surfacer available. They are still in use and offer excellent adhesion to bare steel. The only drawbacks are that they are a 1K product that is forever soluable, if you wet the surface of dried etch primer with solvent it will wash right off. Etch primers can be coated with epoxy and or urethane surfacers. Some newer polyester primers are claimed to be compatible with self etch primers but when I tested this it showed a very slow cure and definitely softened the etch primer underneath.
The only time I use a self etching primer or etching wash primer is on bare metal surfaces that cannot be prepped properly for epoxy primer.
IMO your best bare metal primer is epoxy if applied over a sanded and clean surface. Epoxy needs texture for proper adhesion and most manufacturers suggest sanding the bare steel with 80-180 grit. A sandblasted texture is also ideal. Epoxy offers excellent adhesion and the best durability in a 2k product that is not soluable after full cure. Any polyester or urethane product can be applied over epoxy.
1. sand and clean your bare metal surfaces
2. apply two coats of epoxy primer and let dry overnight
3. do any polyester filler work over the epoxy, if the epoxy is over three days old scuff with a red scotchbrite prior to filler application
4. apply two more coats of epoxy to seal your filler work and also any bare metal cut throughs
5. apply the surfacer of your choice either polyester or urethane
6. block sand the surfacer
7. some surfacers provide an ideal surface for paint while others are absorbent and will need to be sealed.
There's many steps to doing this right, more time than the average body shop will spend. Most will just mud, prime, and paint. JMO's Bob