In general, cleaning cast iron grates with a grill brush follows the steam cleaning method. This is how it works: The grill is preheated at a minimum of 400 degrees Fahrenheit for at least 15 minutes. Then the grill brush is submerged in water before scrubbing the grill brush onto the hot cast iron grates. This process removes the gunk and grime from the grill gates and sanitizes its cooking surface.