General advice is do not clean coins. The main reason many coins have been ruined by poor or aggressive cleaning If you have found a gold coin you are thinking of selling, definitely do not clean it, leave it to the buyer. the buyer, whether a dealer or collector may prefer to buy an uncleaned coin in which case he may pay more for it. If he prefers it cleaned, he will probably know how do it. Why do extra work and risk getting a lower price?
Avoid Abrasives & Rubbing
One of the worst things you can do to most coins including gold ones is to use an abrasive. Most metal polishes, such as Brasso, contain abrasives. these put tiny scratches on the surface on the coin, most collectors and dealers dislike scratched coins, or ones which look polished.
One other source of abrasion is rubbing or wiping coins dry after cleaning them. It is very easy to put scratches on your coin by any form of rubbing. If any grit is present, it will make scratches. Even an apparently soft cloth can leave tiny scratches on the surface of coins. Because gold is quite soft it is easily scratched. If you need to dry a coin after cleaning, make sure you dab it dry. If you have a steam cleaner or some other air blower such as a hair dryer, this may help, although it is still possible to leave patchy drying marks on the surface of a coin.
Best Way to Clean Gold Coins
If you have a gold coin which is dirty, it can be safely cleaned using soap, or detergent, and water. Sometimes when we get a particularly grubby batch of gold sovereigns, we put them in a pan of water, give them a squirt of "Fairy Liquid", and leave them boiling gently for as long as it takes. Don't allow the pan to boil dry or you may find all the coins have turned black, and have burnt-on soap residue on them. Dry them gently as described above.
If you have an ultrasonic tank, you can use this. Ultrasonic tanks work by gently vibrating the coins in soapy water, often a proprietary cleaning solution. Because the action is gentle, it should not harm your coins. It is very similar to boiling in soapy water but the sound waves provide the agitation to loosen any dirt instead of the boiling action.
Stains can be removed. How depends on what has caused the stain. Solvents such as acetone or nail varnish remover will work well for removing many stains. This will also remove adhesive tape residues, it is surprising how many people use Sellotape to stick coins to a piece of paper or card either for storage or to send through the post. Obviously it is best to avoid sticking anything to your coins, as it will often leave a mark which may disfigure the coin.
Rust Stains and Red Spots
We sometimes see gold coins which have rust stains on them. Mention this to most people and their jaw drops open, because they are about to say that gold doesn't go rusty. Of course not, but it sometimes gets stored in steel boxes in safes or deposit boxes. If these are then stored in damp or humid conditions, the box may rust, and this can stain the coins if it comes into contact with them. The rust stain can be dissolved using almost any acid, even quite weak one such as lemon juice, vinegar, or the carbonic acid present in fizzy drinks. Obviously these may leave the surface of the coin sticky, but this can be rinsed off. If rust stains are persistent, you could try using a stronger or more concentrated acid (strength and concentration are not the same thing), such as we describe in our page about red spot.
Because proof coins have a very highly polished surface, it is easy to spoil this finish by careless cleaning, in fact almost any contact can detract from the near perfection of a proof coin. Our comments about drying carefully and avoiding rubbing are particularly important for proof coins. As you will know, even touching the surface of proof coins, will usually leave a greasy fingerprint. If you are trying to remove fingerprints, a solvent such as acetone should work. Obviously it is better to avoid fingerprints in the first place, and if in any doubt, its probably best to avoid trying to clean proof gold coins.
If using solvents, do be careful to work in a well ventilated space.
Courtesy - taxfreegold.co.uk