If I understand you correctly, you're wanting to paint directly on the unprimed metal, but I don't understand what benefit you expect from that which a primer wouldn't improve on. A metal primer would still give you a smooth surface of the metal, and even acrylic dispersion ground on top of that (although, not required) would still be very smooth. Here's a page of an FAQ at the Golden site that makes some recommendations you might consider:
http://www.goldenpaints.com/technica.../xvi_metal.php