Etch primers are convenient for the priming of all types of metal that require a very quick turn-around time. They only require a very thin coat in order to etch the metal surface and create a tenacious bond. Zinc phosphate pigment offers some degree of corrosion protection. The fast dry characteristics allow the topcoat to be applied shortly after primer application.
Both Epoxy Primer and Self-Etch primer (Self-Etching primer) can be used on bare metal. Both provide corrosion protection, and both can be top-coated with primer surfacer.
If you’re doing some body work and you have some bare metal, you don't want to just put regular primer surfacer because it does not have the corrosion properties like Epoxy or Self-Etch.