Pour 2 tsp. of baking soda on a damp cloth.
Rub the baking soda onto the burn mark on the microwave door.
Rinse the mark with water.
Cut one onion in half, and rub the burn mark with the flat side of the onion. Then rinse with water.
Spray the burn mark with ammonia, if the mark remains.
Scrub the mark again with a soft cloth.
Wipe the interior door down with water to remove all ammonia. Then repeat if necessary.