Metal surfaces require specialized primer and paint for good paint adhesion. However, if you want to paint over prepainted metal, you're in luck. Prepainted metallic surfaces can be treated almost like any other basic latex or oil-painted surface. Before painting, you'll still need to do a little bit of specialized prepping in case of rusting, but the main process is fairly intuitive.
Things You'll Need
Scrub any rusty surfaces with a scouring pad. Painted metal is typically well-sealed and does not rust easily, but joints or unfinished edges can sometimes oxidize and rust. These surfaces need to be scrapped clean before painting.
Buff any previously rusted surfaces with 300-grit sandpaper.
Apply a coat of rust inhibitor to any areas where you noticed rusting. Rust inhibitor helps re-seal the surface for better paint adhesion and future rust prevention. Inhibitors are available in spray cans, or they can be applied like regular primer or paint.
Tape any edges or special details with painter's tape if you don't want these surfaces painted.
Apply latex paint to the surface. Acrylic latex provides the best coverage on prepainted surfaces. Use a brush, roller or spray can depending on your preference or the size and shape of the metal object. The paint should adhere to the prepainted metal surface.
Wait for the first coat to dry, and then paint additional coats if necessary.
Remove the painter's tape after the final coat is fully dry.