I purchased a flat top grill and after lighting the grill and the grill sits for a while with out cooking anything on it, it turns black. When I do cook on it during my rush it becomes caked and hard to clean to get a good sear on the meat. It more or less steams the meat. Is this because of the type of metal the flat top is made of? What can I do to prevent it from turning. I clean it with grill cleaner and a scrubby as directed by the manufacturer.