The idea of a rusted microwave oven may seem scary, with all the heating waves bouncing around, but a small amount of rust actually doesn't normally affect the conductivity of the machine, according to the How Everything Works website. However, if the rust feels hot after running the microwave, it may eventually cause the oven to leak. Also, a lot of people just don't want rust fouling up the clean, white look inside the machine. You have to identify what's corrosion and what's just grime before you can clean rust out of a microwave.
Mix 1/2 cup water and 1/2 cup white vinegar in a microwave-safe container. Cook the mixture until the liquid reaches a rolling boil. This boiled solution loosens food stuck to the inside walls and eliminates odours.
Remove the container from the microwave. Scrub the inside of the oven with a damp rag to eliminate any baked-on grime and to positively identify rusted sections.
Sprinkle a generous portion of table salt over the rusted areas.
Cut a lemon in half. Squeeze the lemon and drip the juice onto the table salt until it becomes saturated. Allow the salt and lemon juice to sit for three hours.
Scrub the salt and lemon juice using the cut lemon. Wash the area with a damp rag after removing the salt with the lemon.
Examine the area for signs of any remaining rust. Repeat the process if light rust stains remain, or gently scrub the area with fine steel wool. Wash the area with a damp rag again if you decide to use steel wool.
Make a thick paste of salt and lemon juice if the rust is on the walls or ceiling of the microwave. Spread the paste over the rust and allow it to sit for three hours. Commercial chemical-based microwave rust removers are also available, but can be expensive.