Remove the garage door's power source prior to repair. Unplug the power cord for the door or switch off the garage's power at the building's circuit breaker.
Apply dish detergent to the rusted panels. Scrub the rusted areas vigorously with a soft brush.
Clean the detergent and rust from the door using a rag dampened with warm water. Examine the door for remaining rust damage.
Sand any remaining rust with a coarse-grit sandpaper until all rust is removed. Start with a coarse-grit sandpaper, and then switch to a fine-grit sandpaper if a smoother finish is desired.
Apply a rust-inhibiting primer to the repaired areas. Check that the primer is compatible with steel and rated for outdoor use.
Zinc chromate-based primers are a common outdoor metal primer. Allow the primer to dry.
Paint the repaired area to match the existing garage door color and gloss. Allow the paint to dry, then restore power to the garage door.