Ive been using this technique on my heavy stubber and a sponson mounted heavy bolter, and It works just great. Use this technique when you want to make very worn out or grimey metal.
(I would post some pix showing the steps in progress, I but I accidently left my starter pot of boltgun metal open for a night. )
1) Prime the mini (doesnt really matter which color you use)
2) Paint the metal area you want to look 'grimey' with boltgun metal
3) Stipple on some Brazen brass (or beaten copper, or tin bitz) randomley around the boltgun metal area
4) Give it a shitload of 50/50 black/ vermin brown washes, and finally a 'pure' vermin brown wash
I used it on this heavy stubber dude and this sponson mounted heavy bolter, and It looks great.
(the Aquila thingey was painted later)
Any thoughts, constructive criticism, or ideas are welcome.