I will have some parts that will be taken down to bare metal, what is the best primer to use?

etching, then a good filler primer like duponts 2k urathane

I agree with ragtop, etching primer first then a urethane primer like Dupont uroprime or PPG k38.
But on things like a core support stuff that you WONT be blocking to a perfect flat surface, use the etch primer then instead of urethane just go with a epoxy primer for a sealer then paint color of choice.

You go with the epoxy because the urethane would need to be sanded before painting, were the epoxy you dont. Stuff like the core support,frame whatever you are'nt needing that perfect flat finish like you want on exterior panels so you have no need for a urethane Uk primer because you need to sand that primer before topcoating