I just purchased war machine protectorate of menoth all in one kit. On the big battle robots I want to have two chipping effects. One revealing steel and the 2nd one rust. My plan was to base cost it bare metal hair spray it pain over it then some how I have to figure out how to get a rust chipping too. Should I just base coat metal on the main body an then around arms neck and feet of the war machine base coat rusts? Assuming rust would build up around edges and joints. Or would the rust be better added on as a dry pigment over the metallic chipping?

Both methods are sound, although I'd use the pigments more for stained rust around a paint "blister" which you can replicate with petroleum jelly. Or also for streaking the rust if it's appropriate.