Cleaning coins is fairly risky. Botched cleaning jobs can make a very valuable coin almost worthless on the market, and actually do more harm than good to a coin. Deciding whether or not to clean isn't easy. Cleaning goes in and out of fashion like any other trend; some years, cleaned coins are popular and other years, they're not. Overall, serious collectors prefer "natural" coins.
You will often hear collectors talk about "toning". That's just a nice way of saying tarnish. Oddly enough, an evenly toned coin with an attractive color can be worth more than an artificially shiny, polished coin. Toning, however, is not the same as dirt or corrosion. Leaving these on a coin can cause even more damage to it over time. Removing the dirt and corrosion will often leave pits behind in the surface, but at least there will be no further damage.
A good example of corrosion that should be removed is damage from PVC. PVC (Polyvinylchloride) is a chemical found in older plastic holders that leaves a slightly scummy discoloration on coins over time. This scum can spread so it is best to remove it as soon as possible. Even slabbed coins will have it sometimes.
In any case, you can never hope to get the mint finish back. It's a microscopic thin layer of metal that is worn or eaten away very easily. The best you can hope for is to stop and remove any existing corrosion yet retain the toning.
So, how can you do this? You can either use a solvent specially designed to clean coins without affecting the toning, or you can use "home-grown" methods.
When I upgraded the album setup I was using to store my collection, I realized I had made one big mistake: I put the coins directly into the album pages without putting them into any sort of flip first. The album pages were made of a cheaper grade of plastic that contained PVC, and over time, the PVC had left a scummy film over the coins' surface. Before transferring them into PVC-free flips and putting them into the new album pages, I decided to try cleaning them with a solvent just to remove the PVC.
The first brand I tried was Uni-Solvent. It came in a 4 oz. bottle. I dipped the tip of a cotton swab into the bottle, then wiped the surface of each side of the coin and the edge. Coins that hadn't been in the old pages for very long didn't show much of a change. However, for older coins or ones that had been in the album a while, the change was dramatic! It didn't affect the toning, but it did occasionally remove corrosion or stuck-on materials. When there was PVC present, for some reason it turned the cotton swab a bright blue-green. I noticed it seemed to work best on nickel-based coins. It did alright on brass ones, and didn't seem to have much effect on aluminum coins. It did create odd blue-green tints on copper coins.
While overall it worked out well, Uni-Solvent did have its disadvantages. The warning label on the bottle cautioned that it needs to be used with plenty of ventilation, and that's certainly true. The smell was fairly strong and unpleasant. Uni-Solvent is also a little thick, and it took a few seconds to evaporate if you wanted to air dry the coin before putting it into a flip or album page pocket. It's also difficult to find at the moment. When the bottle ran out, I had a hard time finding a supplier. Most of the suppliers I tried were either sold out of it entirely or only had the 16 oz. bottles.
I actually received Koinsolv by accident when a coin supplier sent me the wrong order. Since I had had such a hard time finding Uni-solvent and I wanted to continue transferring my collection to the new albums, I decided to just try this rather than wait any longer. It turned out to be a happy accident; I was very happy with the product.
It does just as good a job as Uni-solvent at removing PVC and some kinds of corrosion and stuck-on materials. Koinsolv is lighter than Uni-solvent. It evaporates faster and doesn't smell as strongly. Since I had a 16 oz. can of it, I couldn't dip a cotton swab into the can. Instead, I poured just enough into a shot glass so that I could wet a cotton swab with it. Half a shot glass's worth would last me for quite a while before I had to pour out more. For mint-condition coins, if you don't want to run the risk of scratching the surface by using a cotton swab, you could get a shallow dish and a pair of coin tongs. Pour enough of the solvent into the dish that you can use the tongs to submerge the coin in solvent without it touching the bottom of the dish. Swirl it around in the solvent, then wave it back and worth in the air to dry it without touching the surface.
Although Koinsolv is the more expensive of the two, I highly recommend it. As with Uni-solvent, be careful using it on copper coins, as it may discolor them.
Try these at your own risk; I am not responsible for any coins damaged by cleaning!
I searched through six months of archived postings to rec.collecting.coins on Google Groups about whether or not to clean and what methods to use. Here's a summary of what I found:
Here is another method I gleaned from all those postings:
"Put some aluminum foil on the bottom of a pan. Dissolve baking soda in hot water in the pan. Put the coin in the pan so it touches the aluminum for a couple minutes on each side. Rinse and dry."
The main thing to remember is, if in doubt: don't do it!