Wash the drain pan with warm water, liquid dish-washing soap and a sponge. Rinse the drain pan with water.
Dampen a piece of steel wool with soap under a faucet, squeezing the pad to create suds. Scrub the inside of the drain pan in one direction only.
Rinse the drain pan thoroughly and dry it with a soft cloth. Replace it under the refrigerator or wherever it belongs.
Put on latex gloves to protect your hands.
Rinse the rusted drain pan with water.
Apply oxalic-acid powder to the drain pain. Wet a nylon scrubbing brush, dip it into the powder and scrub to remove rust.
Rinse the drain pan thoroughly. Inspect it for any stubborn rust stains and repeat if any stains remain. Dry the pan with a cleaning cloth once it is free of rust.