Brushthralls had an interesting take on this; with metal models, you can try painting rust on the model, sticking rock salt to your models with a drop of water, spraying over in flat black, then flaking it off after it's dried overnight - shows rust coming through from underneath.
Alternatively, work from a black undercoat. Drybrush metal over all edges prominent surfaces - then work up 2-3 layers of colour (for instance, dark brown through desert yellow and bleached bone) in open spaces, leaving the edges as exposed metal, giving the impression of weathering on the edges and raised areas. Then give the metal a bit of an inking with brown, giving it a grimy/rusty feel (or black for an oily feel).
You should end up with a sort of metal-dark-medium-light progression towards the coloured areas from the edges, which gives the impression that the weather has worked through multiple layers of paint.