Though they've been discontinued by Testors, their "Metalizer" line produced very believable bare metal finishes for aluminum, stainless, magnesium, brass, cast iron, "burnt metal", etc. They were used heavily by aircraft modelers because of their outstanding potential to achieve visual realism when applied correctly, and buffed if required.
If you can find any, I suggest stocking up before they're gone the way of the Dodo.
Alclad makes "metal" finishes, as do a few others.
You can get useful information by searching the internet for people who have used all the materials and show their work for verification.
Here's one such link: http://www.clubhyper.com/reference/nmfcb_2.htm