Here's what I did when I refurbished numerous coin doors (Midway, Williams and CoinCo) last summer, as well as the control panels from a MsPac cocktail and Pac upright
1) Sand, Sand, Sand, Sand, Sand - Get it smooth and free of rust and any rough edges from old power paint.
2) Use a metal spay paint primer. Sand it smooth afterwards.
3) Use a Flat (generic) or ULTRA-Flat (krylon) black spray paint. It's fairly hard to make it run. Do a coat, then sand it down with a fine sanding foam block, then do another 3 or 4 coats, sanding between each. Using the Flat or Ultra flat keeps any runs from appearing and gives an EXCELLENT smooth base.
(Between coats, after it is no longer tacky, I set them out in the hot summer sun baking each coat on fully--Warning it gets very hot to touch!!)
4) After I was satisfied with the base flat/ultra flat coats, I then picked out my "final coat". For the cocktail control panels I used high gloss generic paint (of all things.. 96 cents a can). Did two LIGHT coats to ensure no running. Didn't sand between them. Now has a great baked on, 6 layer thick black finish.
For the pac upright I chose a Satin finish Rustoleum paint to more closely match the original power coating. Again, 2 LIGHT coats since it was just to give it the sheen that the flat couldn't give. By doing so many layers of flat, you build up a good color base that is harder to ding-out and expose the metal, while not having to pour as much of a final coat on, risking runs/etc..
For the coin doors, I chose a Semi-Gloss Krylon paint to give it just enough shine but not too much, like the newer powder coated doors.
In summary: Start with a sanded base, put on a good metal primer, sand smooth, then as many coats of a flat base as you can (to keep from creating runs), sanding between, then pick your final coat sheen that you want and apply TWO light coats to avoid runs.