I am a Coin Dealer, and I do know something about coins.  I've been in the business a long time, over 40 years, well known throughout the West Coast.   Business name--Coin Connection Central, PO Box 2332, Covina, CA  91722 -- Cell:  626-589-7498!

I too was once a kid, oh yes, I was.  I am a kinda of a Ripleys Believe It Or Not!, for that time.   A coin collecting kid - nerd, even when it wasn't 'fashionable' to do so, eventually turning my hobby into a full-time business.

Seasoned collectors know not to clean their coins, but I have found that the general public needs a bit of help in this area. 

Now, that I  have been on eBay for over eight years and, have dealt with thousands of people here on eBay-and offline (at Major Coin Expos, etc.), I thought I would throw in my Fifty Cents worth (no pun intended). 

For instance, if you had an antique chair, and it needed some major overhauling, (such as - sanding, new nails, wood putty, fresh coast of stain (etc.), you would do the job, and end up with a "new and improved" antique chair.   More than likely, the people at the Antiques TV Show would scold you, and tell you that this now wonderful, pristine chair (or so you thought it was), would have been worth $2,000.00,--remember the word 'wood',  but with the great "improvements" that you have done with such loving care,  it should now bring around $100.00 bucks at an auction.

Well, coins are the same way, and you can drop their value by trying to "make 'em pretty".  We actually had one lady that stopped by my table at a Coin Expo with a wet baggie full of old silver coins stating she wanted them nice and shiny so she could get the most money for them!!??--I just about fainted!

On another level, some coins can be and are improved with some restoration, however this should only be done by a Michaelangelo, or another specialist in the field.  

Not all cleaning is BAD!  Sometimes it can be the only way a person can afford a particular piece that they have been searching for to fit in their collection, with the hopes to upgrade!  JUST MAKE SURE IT IS GENUINE--and you won't lose!  Remember the word geniune, not grade--any respectable dealer will, and should guarantee geniune!!  If he/so states. 

All in all, if you feel you have a coin that is no so attractive looking,    PLEASE go to someone with expertise in this particular field, and have them guide you.  The coin you save may be your own! 

Best of luck, and good wishes!

Doc