Etching primer contains acid to adhere to the metal and retard corrosion. Epoxy can be shot directly over metal and does provide a good corrosion protection. If you want better, just apply the etch under the epoxy. Some brands of paint have a recommendation to apply etch over CURED plastic filler others don't. Applying a 2K urethane primer over an etch is my personal undercoat of choice. The urethane is a perfect surfacer for blocking and such and the etch provides the corrosion protection. Epoxy is very poor for surfaceing. If the item is not going to be surfaced then epoxy works great, like on a frame or a trailer.

Again, some brands have a recommendation to spray paint directly on top of the etch prime. Some say only certain paints.

What brand etch and paints are you using.

1948 Chevy Pickup
Chopped and sectioned
owned since 1974 when I was 15.