If you are painting the interior with a metallic, rule of thumb would be to use a 2-stage. I ignored the rule of thumb and shot my interior with a single stage metalic, for me it turned out pretty nice. I work with guy who was in the business, and he said no, you don't "have to" use etch primer on bare metal when using a epoxy primer. It may produce better results to add the step and use a etching primer but it is not critical to paint adhereing to the metal. Epoxy primer works and then be sure to add a seal-coat primer over the top, no matter what you use. I don't know how to do a nice job, but I can tell you what worked for me.
