Simple question, when do you use etching primer and when can you use regular primer, like epoxy or urethane?

etching primer is for bare metal and regular primer is for painted surfaces or body filler etc.

Epoxy primer is also a direct to metal primer. When reduced it also acts as a sealer. My understanding is epoxy is water-proof thus helps prvent any future rust but it must be top coated as it does not have much UV resistance. My understanding is it can also be difficult to sand. Normally you would use your epoxy or etch primer on the bare metal, followed by your regular primer (2K is better than laquer). Your filler can be applied over epoxy primer btw...

Epoxy is about the best thing to use,as stated above,we coat a bare metal car with it,then do bodywork over that. It seals the metal off from any moisture,and protects it under the bondo,if any moisture gets into the bodywork,through a chip or something,it will not get to the metal and start to rust it. The etching primers are not even used at our shop anymore. The urethane primers are high build,for over bodywork,and will absorb moisture over time,and are NOT weatherproof!!