Doing a google search for painting on metal give back the following advice:
1 - always always always use a (metal specific) primer (it doesn't hurt to give it several coats before the actual job)
2 - (before 1) use a fine sandpaper to get rid of grit and rough up the surface to bond with the paint better
3 - use acrylic paint (again several coats) that are suggested for metal use
but a bunch of people here have actually done paint jobs so if they say anything different, don't listen to me
