I'd start with a Brass Scorpion base coat, then a heavy drybrush of Sycorax Bronze. Then give it another drybrush of 50/50 Sycorax Bronze/Runefang Steel to give it the almost steel/silver look. Don't worry if it looks a bit too silvery at this stage as the heavy wash of Agrax Earthshade will bring back the browny/bronze colour. Then a little Nuln Oil in the parts you need darker, like around rivets and skulls. Maybe another drybrush of Sycorax Bronze/Runefang Steel around where you want the details to pop.
Thats how i'd do it, hope that helps.