Barry, I would suspect that if cleaned with something like baking soda and water, it perhaps leaves microscopic scratches on the coin, or at least dulls off some sharp edges perhaps to indicate it's been cleaned. If I don't plan to sell a coin I clean it with a bit of baking soda and water rubbed between my fingers. Works great. If it's got a real stubborn stain on it then I whip out my home made electralysis machine (wall DC transformer using stainless steel hemos for the two electrodes and a glass of salt water). Cleaning with electralysis though is far more risky than other methods, because if you over do it you can actually start to stain the coin again, or if you over do it you can even pit a coin (usually copper or bronze coins are at risk for this, but perhaps silver too?).
I would try their peroxide method, and not baking soda or any form of polish, as they usually contain some kind of grit. You want to remove the tarnish but not via any abbrassive action.
And yes, once cleaned, then here's a tip I heard to fix it so somebody can't tell it's been cleaned (so long as you didn't polish it or leave micro swirls on it or scratches)- Set the sucker outside somewhere where it's subject to rain and weather. Keep an eye on it, and flip it over about once a week too so both sides are equally exposed to the elements. Don't stick it on any kind of metal surface where it might react with it. Instead, something inhert, like perhaps a glass patio table or a cinder block or something.
The way I look at it I think the idea of a coin being cleaned making it less valuable is insane. I don't want to look at a dirt old silver coin myself. Rarely have I seen one that I liked in that condition at a dealer. I prefer nice and shiney myself. But, if everybody has been brain washed into believing that cleaning destroys the value, then that's the way it is. So, if you can clean it, and the let nature outside age it a bit again, and nobody can tell it's been cleaned, then I don't see that myself as any form of falsehood. After all, they are examing the coin, and if there are indicators of a cleaned coin, then they'll see them, and if there are none present, then what harm is done?
Incidently, I cleaned a seated quarter in mint condition, only because I put two nasty dings in the side when I dug it (DOH!). I figured I'm cleaning this puppy, along with a silver 3 cent piece in mint condition too, because I had no plans to sell them anyway. That quarter was about a $1400 coin if I hadn't dinged it. Have no idea what it's worth now cleaned and dinged. The 3 cent piece was about a $200 to $350 or so coin if memory serves, but again I cleaned it with no intentions to sell. On the other hand, I dug a 1921 standing liberty in F-12 condition. It wasn't even stained but I had no intention of cleaning it anyway because it's really clean already, and it turns out it's about a $350 coin or so if memory serves, according to what I've seen them in that condition and that year on Ebay going for. Somewhere in the $350 to $450 range or so perhaps.
Let us know what you do with that coin, and show us what happens when done. I'd clean it the peroxide way, then age it outside a bit, and then have it graded and if it comes back cleaned then you knew what you were getting into, so no harm done. I couldn't imagine it being worth more in the above condition that it would be cleaned myself. Good luck...
Take out, give it a *gentle* terry-towel persuasion, and repeat as necessary. You might also try a solution called Ezest.
