Another technique is to paint the item in its representative metal colour first (chrome, brass, gun metal etc etc), once properly dry, seal that with a clear/top coat.
The idea is to mask off all the scratches/chips and the like, then give the item its proper paint job leaving the masked areas on.
A favorite mask of mine is latex, I can use all my paint brushes with it including fine grades (as long as I wash them before it cures), and a hairdryer speeds things up considerably. Others have used hairspray to good effect, vasaline is also used, though I've not bothered with that one as its a pain to cleen up after.
Each produces different types of results, it all depends on how uou see the item being used to have aquired the weathering you give it.
Once you have given the item its main paintwork, and prior to washes, remove most of the masks from the item to reveal the metal paint job underneath. I like to leave one or two till the very end after ive finised weathering it as washes will dull the metal paintwork some (older chips/scratches), so by leaving one or two till then, they look like new chips/scratches :thumbsup
Here is a pic of a hand dryer cover I did for a friends steam punk café using the above technique: