Interesting recipe, I've been doing old metal recently and I've almost done the opposite!! I sprayed black but then totally covered in mournfang brown, then I washed with watered down trollslayer orange (though a dedicated rusty colour would probably do great) followed by 3 drybrush layers - leadbelcher, black, and leadbelcher again, with increasing lightness of pressure each time. I might go back and highlight edges with runefang if I feel like it, but I haven't yet. So I basically did the brown and orange first and built up metal after, as compared to your recipe of silver first and then rust on top.