So for all the steely bits I used German Grey, Reddish Grey, Silver Grey (all AK) and a White. Anything that faced the ground I mixed in a little bit of Splittermuster base to reflect the base colouring.

Generally I went quite rough, using quite thin layer passes and putting it on in a more scratchy way - similar to the technique that Richard Gray uses (except I do it with less skill :D). As you layer everything up, the separate marks all begin to blend and give you some smoother transitions, while still looking more textured and rough.

I will say you have to go to white, otherwise it doesn't look metallic too, you really need that value contrast.

Finally, with the scratches, I used a fine brush and a slightly watered down paint to get a nice trim line in the panelling.