I have the bi-tone sub too and it also started to rust. I was CCing it. The blue wonder cleaner does a great job getting the rust off. Then I gave it a nice coating of blue wonder armadillo and I haven't had a problem with rust yet. OR, you could just get it hard chomed, which will protect from rust and make it look badass.
My Bi-tone slide has discolored!?
I went to a considerable amount of trouble and extra time as well as higher cost in obtaining a Bi-tone sub. Imagine my chagrin upon discovering that it has started to rust! I have kept it oiled as much as any of my other fireams and was not expecting this at all. If this finish is hard chrome, it has to be the thinnest application I have seen to discolor this way! It has spots in the cocking grooves as well as along side the front and rear sights. 
