Diagnosis
The interior has a stainless steel or coated surface that can be scratched by abrasive cleaners or tools.
How to Fix It
Use a soft cloth dampened with warm water and mild dish soap. Wipe the interior gently. For stubborn stains, place a bowl of water with lemon juice inside and microwave on high for 2-3 minutes to steam clean. Wipe with a cloth.
If That Didn't Work
For baked-on grease, use a non-abrasive cleaner specifically designed for microwave interiors. Apply with a soft sponge.
Heads Up
Never use steel wool or harsh chemicals. Avoid getting water into the vent openings or control panel.