Etching primer is a type of primer paint that has a high acid content that allows it to form a superior bond to bare metal. Especially bare metal that has been exposed to the elements for more than a day or so. An even, heavy coat of etching primer will form an excellent base coat that will allow one to either paint over with a regular primer right away, or wait a couple of days before adding any additional coats of primer. Epoxy primer is also a very high quality primer that works very well as a base primer coat for future coats of primer.
For more info, see Self Etching Primer vs Epoxy Primers