An etching primer is used for car body work. Etching primers both etch and prime car metal to prepare it for painting. Self-etching primer is made for bare metal applications, and should not be used over paint. Some people prefer epoxy primer over self-etching primer, although both have advantages. They can be used in tandem.
Self-etching primer is made to stop or slow the spread of rust. Before using the primer, remove all rust that is currently on the auto metal. The primer is made up of zinc and acid. This substance chemically reacts with the bare steel on a car and impregnates the steel with primer. This makes it harder for water to get to the steel, thereby slowing or stopping the rusting process.
Order of Application
Self-etching primer is sprayed on any bare metal that is showing during a car body repair. After removing rust and prepping an area, spray the self-etching primer on any exposed metal, including screws and bolts. The regular primer goes over the self-etching primer.
Some people prefer an epoxy primer to a self-etching primer. Modern formulations of epoxy primer have similar rust-inhibiting qualities and are more versatile. All rust and old filler materials should still be removed when using an epoxy primer.
Self-etching primer is usually sold in aerosol spray cans. Epoxy primer is a two-part primer -- a resin and an activator part. You should prepare the metal surface well before applying the primer, and spread the resin evenly before applying the activator.