I think the best looking paint I’ve used for this was Pactra metallic gray. Unfortunately I believe Pactra is out of business . So I mixed some dark gull gray with a little Testor’s steel and a touch of black. I put a BB in the bottle so it stirs up real good when I shake it and I’m pretty happy with that.
I use a combination of things to achieve the metallic look. First I brush on Tamiya Gunmetal. Then a light drybrushing of steel following by burnishing with graphite. The graphite really works well and gives a metallic finish to the part.
I used Vellejo Black Gray for a base coat on mine, then drybrushed with their Steel on the edges and wear areas. The rest of the weathering was done over that and I thought it came out pretty good.
Adam, you’ll get as many different opinions as you get replies on this one.
I like to base coat with flat black, then wash with a burnt sienna and tan mixture for rust (if appropriate), then dry brush with Testor’s Steel and highlight with Testor’s silver. But I’ve also used Vallejo and Tamiya Gun Metal. All seem to work well.