As mentioned, the Model Master Metalizers are excellent (my personal favourite) airbrush paints for replicating bare metal finishes, but the entire Model Master line has recently been discontinued. There are some other options. Alclad has a line of metal paints that look very good, and like the MM Metalizers are intended for airbrush use. Vallejo carries a line of metal paints in acrylic that seem quite popular, though I haven't tried them. Two lines of metal paints is offered by AK Interactive; the True Metal series and the Extreme metal series. The True metal is a thick paint in a tube, and the Extreme Metal is an airbrush ready paint. I haven't used any of the AK paints either, but I'm quite curious about them and will be buying a set soon.