After a few minor 'incidents' where the grill was left on accidently with the door closed, the front of my cooker (where the knobs are) is really black/burned. Is there anything I can do to clean it? Various sprays, magic erasers haven't worked, and it looks awful. Can't afford a new cooker, but am totally fed up of looking at existing one in such a state. Is there a magic way that I haven't found yet?