Rust is one of the worst four letter words in a motorcyclist's vocabulary. Although some riders may be fortunate enough to live and ride in an area that does not easily promote oxidisation, the reality is that rust can and will happen. One of the first components on a motorcycle to show signs of oxidisation are the brake rotors. Most of the rust that forms on the rotors can be removed just by riding and braking normally. But to remove the rust that forms at the edges and other places the brake pads can't reach requires a little more effort.
Clean the brake calipers by spraying lightly with brake cleaner. Immediately wipe away the cleaner with a rag to remove dirt and oil build-up.
Clean the edges of the rotor by scouring lightly with Scotchbrite. Once the edge is clean, scrub the inner hub of the rotor with Scotchbrite. Work gently to avoid scratching the finish on the inner hub.
Prepare a solution of mild dish soap and water and apply to the rotor with a clean, soft rag. Thoroughly rinse the solution away using clean water or denatured alcohol, then dry with a separate clean rag.
Repeat the process to clean the other rotor.
If possible, raise the motorcycle off the ground using a motorcycle stand to help you rotate the wheel and rotor. If a stand is not available, you may need to roll the motorcycle forward or backward slightly to access the entire rotor. Apply Scotchbrite to the rotors using a gentle, consistent pressure to avoid scratching it. If you will be working to remove rust from the face of the rotor, try to work along the "grain," matching the circular pattern left by the brake pads, to prevent irregular wear of the pads. If you doubt your ability to complete this project, have the work done by a qualified technician.
Do not, under any circumstances, use WD-40 or similar products on your motorcycle's brake rotors. WD-40 has lubricating properties that will impair your motorcycle's braking capabilities, possibly leading to a crash or injuries. Be sure not to spray any painted parts with brake cleaner, as the cleaner will eat through paint easily. Brake cleaner can be dangerous if mishandled. Always wear gloves and protective eyewear when working with chemicals.