Rotors are subject to corrosion just like any other metal component on your vehicle. Over time, brake rotors can develop a rust film on their surface. This usually happens when the vehicle is driven in wet or humid conditions and then parked for an extended period of time without being used. The rust film will typically not cause any problems with the operation of the brakes, but if it becomes too thick it can affect braking performance. If you notice that your brake rotors are starting to get rusty, you can remove the rust film by sanding it off with fine-grit sandpaper.