Scuff it up with a green Scotchbrite sponge or pad, merely to knock the gloss off the paint. That is paint. Very important, this step!
Wipedown any dust or grease.
Prime it with a primer that reasonably matches the color of the proper you used on the adjacent walls. This is a very important step. First choice is an alkyd primer such as Rustoleum 7780 Clean Metal Primer (they recently changed the number) or Kilz will do in a pinch. Second choice is a latex primer, but only use this if you are very sure about your work in steps 1 and 2.
Primer is important because paint cans lie. There is no such thing as primer-paint. Background colors DO print through paint, especially the near-white pigments used on almost all architectural coatings. If you want the topcoat to match, the underlying primer color must also match. Primer's job is to provide a uniform surface that is the same in absorption, texture, chemical interaction, and color, so that topcoat comes out equal in all locations.
By applying a separate primer, you are able to force the surfaces to be equal by adding primer coats wherever a background color is still printing through the primer. When the primer job is complete and even, that means the 1-2 coats of topcoat will print through, but they'll print through consistently.