JD,
I've used this (and a couple of their other products) when I did a complete brake job on our '10 Subaru Forester about three months ago. Never have seen any signs of light rust on the rotors from all the rainy days we've been having -- same thing with my wife's MB, and I replaced those brakes several years ago. The chemical stays on the rotor. I found out about this stuff from a guy that sells Porterfield pads for Mercedes and includes a kit with the pad purchase.
GWR
Regards,
paul...