Remove the grill's cooking and charcoal grates from inside the bowl. Coat the bowl of the grill with spray-on oven cleaner. Replace the grates and coat them with spray-on oven cleaner as well. Wait as directed in the oven cleaner's directions. Remove the grates again, and set them on newspaper.
Scrub residue off the grates with a rag, steel wool or a wire brush, as needed.
Wipe out the bowl of the grill with wadded newspapers. Use a high-pressure hose to rinse off the bowl and the grates. Dry the grates and put a very light coating of cooking oil on each one. Remove stains from the lid using warm, soapy water and a fine steel wool pad.