Diggins method id similar to mine. By mixing non-metalics in say grey with silver then you can get a nice fake non-metalic metal look.
I tend to base paint it the colour i want, so for steel I will paint chain mail or bolt gun, the wash it with a mix of black and blue ink, then retouch with the silver and highlight with mithril.
For brasses or gold then its the same kind of thing, but start from a base of tin bits, then a browny green ink mix, then the metalic colour and a highlight of the colour with shining gold mixed in.
You can play around with the ink wash colours, purple makes things seem a bit magically, blues for strong, brown on silver give an aged effect ect.
Really they are no different from non metalics, but I would usually leave them until last and use a separate water pot so you don't get shiny glitter bits in the other colours, like to woods and stuff.
I don't go in for this metalic self highlighting business, it does look better if you just treat them as you would regular paints.
No washing up liquid this time