Here lies one of thebiggest tips to rule them all. So big that it is in fact staring you in the face - why paint metal on a model made of the actual stuff? Yes - I'm talking about using the metal of the model, rather than paint it. Don't spray up your mini - you can base coat him later. To do this just take a piece of fine sandpaper/wet wet n' dry, and abrade the surfaces of your model which are meant to be metal 'till they are relatively flat. Then get an object like my trusty 'metal-ended' pen (it can be anything with an edged - not sharp - hard metal surface) and scrape in any direction. You should notice a magnificent polished finish is appearing on your metal model, like a superbly polished armour plate. Just water right down some black paint and wash it into the recesses of the to represent metal parts of your model. When it is half dry, rub your finger in the direction that is not against the recesses, once, on the washed areas - to naturally highlight and remove wash from the higher points of the model. You can glaze it to dull it up/colour it if you want from here, but otherwise - it's done! Very simple, time saving, and as real as the real thing!
I tried it out on an Uruk-Hai - a bit shiny for one, but never the less a great test piece. Came out awesome first time!
Anything like this will do! - Use any metal bit for different tasks!