I have 3 large grates so I placed them in a kitchen garbage bag with 2 cups of ammonia. Closed bag with rubber band and placed on rubber mat in my tub. Left over night. Washed off with Dawn and my grates look Great!
ONLY use this and it leaves floor spotless. (Heavy duty floor cleaner recipe: cup white vinegar 1 tablespoon liquid dish soap cup baking soda 2 gallons tap water, very warm.) It leaves everything smelling amazing.