Fill the sink or a large tub with warm water and dishwashing liquid. Place grates in water and clean them with a rag. Rinse with clean, warm water.
Dry grates completely with a soft, clean cloth.
Add a few drops of vegetable oil to clean paper towels and lightly coat all surfaces of the rack.
Add charcoal to the grill or be prepared to ignite a gas grill.
Place grates back onto the grill. Fire up the grill to maintain a steady, medium temperature. Allow the grill to heat the grates for 20 or 30 minutes.
Turn the grill off completely after the allotted time if you are using a gas-fueled one. Let the grill cool before covering it for storage until the next use or add food to the grill grates to begin cooking.