Diagnosis
Grease and food splatter polymerize onto the enamel surface during high-heat convection cycles, making it hard to remove.
How to Fix It
Use a dedicated microwave cleaner or make a paste of baking soda and water. Apply it to the soiled area on the ceiling. Let it sit for 15 minutes. Use a non-abrasive sponge or cloth dampened with very hot water to scrub. The grease should lift. Wipe clean.
If That Didn't Work
For stubborn spots, use a very mild degreaser like diluted Simple Green. Spray on the cloth, not directly into the cavity, to avoid control panel damage. Wipe immediately and thoroughly.
Heads Up
Never use steel wool, abrasive pads, or harsh oven cleaners. They will permanently damage the enamel coating and make future cleaning harder.