You can clean it with Cilit Bang or lemon juice will help and polish it with Tooth paste with a rag, or get a battery charger and a Tupperware container get a strip of Alfoil say 4"x 8" and use one of the lead clamps to hold the foil to the edge of the container with the main part of the foil laying flat against the bottom of the tray and lay the coin on the foil and clip the other clamp on the other end of the tray and fill it with water so it comes just about 1/4 or so up the clamps and turn the battery charger on low or to about 4 Amps on the setting dial and just watch and it will come up like New, Ok.
Do not walk of and leave it cooking on its own, you wont get a belt if you put your fingers in the water but if you have any cracks on your fingers it will tingle a little if you put your hand in there when it is switched on, OK
so don't panic alright, good luck,, John