Most rotors are made from unprotected cast iron, so they are going to rust. The pads will clean the rust off the swept area, but the rest of the rotor can rust. Normally this isn’t a problem, but there are rotors available that have a treatment for the unswept areas of the rotors. I believe Centrix is one of those brands. EBC brand rotors are also treated and are available in slotted and slotted and cross drilled. I chose the slotted for my Saturn.