I would apply thinned grey acrylic paint.(don't forget the drop of dish detergent) This will tone down the paint and give it a faded look. After you are finished with the grey washes and they have dried, then apply a thinned black wash. This will enhance all the recessed lines. After this dry brush gun metal grey/silver over some of the raised surfaces.

This picture of my T-34 demonstrates this technique the best. The other trick is using womens eye shadow. They make many shades that have a metalizer in them. This works well to apply to the edges to make it look like the paint has worn through to the metal.