Metal is tricky in model form. Ideally you'd make whatever it is in the right material and just polish it, but that's not always possible.
At the IPMS last year, I came across Darkstar "Molten Metal" paints and was very impressed. Unlike enamels, the pigment doesn't drop out of suspension all the time so there's no need to stir constantly.
Airbrushes.com now stock the paint, which makes it easier to get your paws on. After arranging for a review for MREmag, I've had a go on the Barclay where I needed to paint the copper pipes and brass valves. While the pipes really are copper, and the brass bit are cast brass, cleaning either up was more trouble than it was worth.
The paint initially didn't want to stick to the smooth, sprayed enamel base but constantly working it solved this and after a few minutes it had coated perfectly. Ideally I should have used a primer base which would have solved the problem.
The results look pretty good to me. For copper, I used copper colour. Brass is "Regency Gold". Both will have a wash of black come weathering time which should make them look even better.