What Pete says. EVERY manufacturer uses a bare metal prep (etch) prior to painting. This conditions the metal surface so that paint will stick as well as removes any mill scale or foreign ferrous metals. .
While all manufacturers use a specific prep product I think it should be noted that they are not all created equal. SPI uses a water born W&G remover with a specific PH. It is not a muriatic acid based converter and they explicitly state not to use acid before using their epoxy. So not EVERY manufacturer uses etch. Metal prep yes.
PPG dplf epoxy can be sprayed using regular old dx330 w&g and does not require a converter if film build is adequate but does play nice with acid converters.
Many epoxies can NOT be sprayed over an acid etch primer for example ( zinc based).
The PH of the metal can be more temperamental to different brands than others. Kirker seems to be more resilient to acid from what you guys have said on the forums. SPI is not.
I was taught to rely on the mechanical bond of the epoxy to the 180 da crosshatch to clean metal and not to rely on a chemical bond. Soda blasted cars need to be neutralized in many cases as the PH of the metal is incorrect. You guys with kirker seem to have found it works well but I wouldn't lump all epoxies in saying they will all play nice with another prep/acid product.