Beware of oven cleaners, which may work but can discolor the racks in the process. Instead, try a product designed to cut through baked-on grime on the cooktop (Affresh Cooktop Cleaner, about $5; lowes.com
). Simply rub down the racks with it, let set for 5 to 10 minutes, then go at them with a scouring pad. Rinse and dry thoroughly before putting the now spotless racks back in the oven.