Lots of options depending on what you want your end result to be...
Sophisticated finishes makes a line of metallic surfacers Steel and Pewter are my favorite colors. Good for a quick application on details or structures.
http://www.patina.com/metallic-surfacers.htmlThey were available at Micheals when I bought mine.
Alcad is the tried and true method for a good finish on passenger cars, not sure which color is best but I've seen pretty good matches to Kato paint...
A bit of a process, but well worth it in the end.
http://alclad2.com/finishes/regular/Testers Testors also has a line of metalizers, that I've had good results with... but I would use Alcad over Testors, it seems to get a superior finish...
also if you're just hoping to represent dirty oxidized metal a greenish/grey primer is a good representation... Unless its taken care of metal stops looking "metallic" pretty quick...