When soot covers the inside or outside of the microwave, a black, oily residue will develop on the surface. In addition, the soot leaves a nose-wrinkling odor inside the microwave. You typically cannot remove the residue with all-purpose cleaners and--if not removed--the soot will cause staining on the microwave surface. A few methods will remove the soot as well as its odor from the microwave. If the first method does not work, try another one until you have removed all traces of the soot.
Place a microwave-safe bowl filled with white vinegar inside the microwave. Cook on high heat for up to 10 minutes until the vinegar begins to boil.
Let the vinegar-filled bowl sit inside the microwave for 15 to 20 minutes. This will give the steam from the vinegar enough time to cut through the greasy soot located inside the microwave.
Dip a sponge in the vinegar-filled bowl. Wipe the soot off the inside of the microwave. Move to the outside of the microwave and scrub the soot off the exterior of the microwave. Rinse the sponge clean when soiled.
Repeat Step 3 until you have removed all traces of the soot. Wipe the inside and outside of the microwave with a cloth damp with water.
Put on rubber gloves. Pour a gallon of warm water in a bucket. Stir a tablespoon of trisodium phosphate in the water.
Dampen a cloth or sponge in the solution and scrub the remaining soot off the inside and outside of the microwave. Rinse the solution off with cool water and wipe dry with a lint-free cloth.