Try rubbing alcolhol and a Q-Tip.the alcohol will evaporate but will cut the grim. Dont scrub so hard that you damage the gold finish on the front. If it does not come clean send it into Hobby services and they can replace it for a fee. It might be a good idea to send it in anyway for a check up if you bought it used. Its good added insurence to have your radio serviced once a year if you do alot of flying. Hobby Services has a great turn around time and only takes about a week. They will e-mail you or call you with with what they think should be replaced first before doing any work. I send mine UPS with a tracking number and they will send you a tracking number when they send it back to you.
NEDYOB