Cleaning up engine bay for paint questions

No doubt about it. DTM or epoxy primer on bare metal or quality primer on properly prepared
painted surfaces.
As for rattle can primer and enamel...most would shudder at the thoughts of that. I hate to
admit it but i have done that with good results that stood up well. Spray cans can be made
up to match your paint and are so easy to use. But you are taking a risk