Primers for paint - questions

Hi all,
I need a primer on primers.
Can you use any sort of primer on, say, an epoxy surface? Or should one use an epoxy primer? What about a wood surface? Metal surface?
Are there certain primers for certain kinds of paint? I see epoxy primers - can I use any sort of paint on that? How does primer provide a good grip on both the underserface as well as the paint?

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