DIAGNOSIS The stained film is baked-on grease and food particles on the interior coating.
FIX Place a microwave-safe bowl with 1 cup of water and 2 tablespoons of white vinegar inside. Run on High for 5 minutes. Let stand for 5 more minutes to steam. Wipe the interior thoroughly with a soft cloth. The softened residue should lift.
IF THAT DIDN'T WORK For stubborn stains, make a paste of baking soda and water. Apply directly to the stain, let sit for 15 minutes, then gently scrub with a non-abrasive sponge. Wipe clean.
HEADS UP Never use abrasive cleaners, steel wool, or harsh chemicals like oven cleaner. They will permanently damage the interior's non-stick coating. Clean the grease filter regularly to reduce splatter.