Microwaves are notorious for being difficult to clean. Any food that splatters on the walls becomes burned-on during the following uses. The best method is to prevent these stains. Cover food with while heating with a paper towel or microwave-safe container. And if food does splatter onto the walls, clean it up as soon as possible.
Removing the Stains
Cleaning the inside of microwave is tough but this simple trick will make life much easier. Mix 1/2 cup of vinegar and 1/2 cup of water into a bowl. Then place the bowl (make sure it is microwave safe) into the microwave and start heating the mixture. Let the liquid become hot enough to boil for 20-30 seconds and then stop the heating mode. Leave the mixture to cool for several minutes.
The vapors from the evaporation process and heat should loosen up the stains. Use a paper towel or nylon brush to clean up the surfaces. If the stains are still tough to clean off, try heating up the mixture again for a little bit longer. Repeat until you are able to clean the surfaces.
This simple trick for cleaning should have your microwave looking good in not time. Also, since the stains absorb energy, the microwave should operate more energy efficiently.