I'd offer help on the weathering but all i know usually is done before you paint it with the finishing coat :( sorry. Orks tend to have a lot of rust, and i find it best to paint copper wherever you want to have rust, hit it with hairspray, throw salt on it, paint over it normally, brush salt off, jab the crap out of it with some Earth pigments. Gives a bit of a chipped paint and some rust feel. Not the greatest but its what i do