Leave coin cleaning to the expert conservators. Never clean a coin yourself until you master proper cleaning techniques.

Most coins should be left alone, but some coins cry out to be cleaned. The following coins could benefit from a cleaning, but they all need to be treated differently:

- Coins found by metal detectors
- Coins that are extremely dark with tarnish
- Coins that have developed a film from spending too much time in a vinyl coin holder
- Coins with tape residue or some other localized problem
We know you have coins that need to be cleaned, and we know you’re dying to clean them. So because you’re going to do it anyway, we may as well give you some safe cleaning tips. (Abstinence seems to be out of the question.)
Please practice these techniques on inexpensive coins until you feel comfortable cleaning coins that are more desirable. Never clean coins that are valuable — take them to a professional instead. Finally, if you don’t feel comfortable cleaning coins, don’t do it.

Removing dirt Dirt is an abrasive that will scratch the heck out of a coin if you’re not careful. Never rub, never scrub, never polish — always use a light touch. 1. Use an artist’s paintbrush to lightly whisk off any surface dirt. Be careful not to exert any pressure. 2. Rinse the coin under warm water to remove additional surface dirt. Don’t rub the coin between your thumb and forefinger.

3. After rinsing the coin, pat it dry with a soft towel.
Avoid pressure and any rubbing motions.Do not use paper towels to dry a coin — the wood fibers in the paper may leave hairline scratches on fine surfaces. Use a 100 percent cotton cloth.
For really crusty and dirty coins, soak them in olive oil for a couple of days and try picking the dirt off with a toothpick. Again, be careful not to exert too much pressure, or the toothpick may scratch your coin.

Removing PVC film
PVC softens hard plastic into vinyl, a popular material in coin holders. Unfortunately, PVC can leach out of the vinyl and onto your coins, causing a light haze, green spots, or an oily film. Over time, PVC attacks the surface of a coin, dulling the luster and etching the metal. PVC contamination is so serious that the major companies that grade and certify coins refuse to grade any coin with visible traces of PVC.
Removing PVC contamination is easy, but it must be done with care. Acetone, the chemical used in nail polish remover, works very well at removing PVC film and spots, but be careful to use it only in a well-ventilated area away from any flames. You can find pure acetone (the only kind we recommend) in the cosmetics section at your local drugstore or in the paint section at your local hardware store.
You may find it necessary to repeat the process with a clean swab until all the PVC is removed .  The first swab will be green in color; the last swab should be clear.
Keep in mind that acetone is a dangerous chemical. Be sure to read and heed the warning label on the bottle.

Removing tarnish or toning
If your silver coin has light toning around the edges, if the toning is attractive, or if the toning is iridescent (showing shifting changes of color), consider leaving it alone. Many collectors pay huge premiums for nicely toned coins.

Nevertheless, if you absolutely hate the toning, you can try dipping your coin in a chemical bath specially formulated for removing tarnish. Your local coin dealer probably carries e-Z-est Coin Cleaner, also known as Jeweluster or “dip” and widely used by collectors and dealers alike. In Ron’s opinion, pure dip is too harsh, so he mixes it with equal parts water to make
a softer solution.
You may use dip to clean nickel coins, but never copper coins. Never dip coins of different metals in the same solution — sometimes one metal will plate out onto the other coin. Instead of cleaning your coin, you’ll be adding an extra layer of metal.

Follow these steps for lightly tarnished coins:
1. Quickly dip your coin in the solution and pull it out just as fast.
2. Immediately rinse the coin in warm water.
3. Use a soft cloth towel to dry your coin.
Pat, don’t rub, the coin dry.
Heavy toning becomes a problem for two reasons:
-The surfaces may have faint hairline scratches and/or marks that are difficult to detect beneath the toning. Before dipping the coin, be sure to examine it carefully to prevent any surprises.
-If the surfaces are too heavily tarnished, the tarnish will have eaten into the coin’s surface. Dip will remove the tarnish but the result will be a coin with an unattractive, dull finish, with no luster.
Practice with a variety of inexpensive, toned coins to figure out what can be fixed and what can’t. Again, when in doubt, leave the coin alone. More coins are ruined by well-intentioned but ignorant coin collectors dipping their coins than by any other method.

Removing lacquer

A hundred years ago, some collectors attempted to preserve the quality of
their coins by applying a thin coat of lacquer. Generally, this method worked
well to preserve coins as long as the layer of lacquer remained unbroken.
However, spots and toned areas resulted when the lacquer became damaged grade a coin that has lacquer remaining on the surfaces.
As with PVC, acetone remains the chemical of choice for removing lacquer.
A short bath in acetone will remove most of the lacquer on a coin, although
additional baths may be necessary to remove all traces. Be patient, and above all, be careful. Acetone is a dangerous chemical even when used properly.

(Abstracted from Coin Collecting from Dummies, 2nd Edition)

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