Here's what I would do...
1) Wipe down metal with acetone or aqueous degreaser (avoid alcohol, as most contaminants you're likely to encounter are going to be polar, for which acetone is more appropriate)
2) Scuff with green or red Scotch Brite pad, wiping with a fresh cloth as you go
3) Before the metal flash rusts, coat with at least one coat of self-etching primer... self-etching will help cut through any flash rust that develops (even the stuff you can't see)
4) OPTIONAL: coat with any automotive grade primer if you want a smoother finish or want to increase corrosion protection. Self-etching primer, as the name suggests, does indeed "etch" the surface; it's great for adhesion but poor for corrosion protection as it typically doesn't have the same levels of zinc as a non-etching primer.
5) Coat with paint of choice - I like VHT Epoxy for stuff like this (
https://www.vhtpaint.com/specialty/vht-epoxy-all-weather-paint)
6) Regardless of whatever paint you use, coat within specific recoat time frame.