Generally, you should never clean coins. Any self respecting collector would tell you that. Many coins loose their value when cleaned and a buyer with a keen eye can tell when a coin has been badly cleaned. Gold and silver coins can be (cleaned) NOT (polished). If you buy a gold or silver coin that is a little grubby then the best way to clean it-if you must- is by using a little soap and distilled water. Tap water should never be used as chemicals are often added to assist purification. These chemicals may well harm the luster of your coin and damage it's value. Never use an abrasive cloth or brush, gently soak it for a couple of minutes then dry the coin by padding it. Never wipe the coin dry as this may cause friction and scratch the surface.
Copper coins should never be cleaned or washed! Many copper coins suffer from verdigris. It's basically rust. A form of corrosion that slowly eats away at the surface. If a coin in your collection has these copper spots then I'd seek professional advice on what to do. Never place them in a self made acid dip. This is not only dangerous to your health but could also ruin your coin. Remember, copper reacts to any type of moisture so keep them coins away from water!
If you have a collection of proof coins that have oxidised then leave them as they are. Never attempt to clean them. These type of coins should never be handled or cleaned. If you do need to take the coins out of their protective cases then make sure you wear white cotton gloves as the acids from your fingers could ruin the surface of the highly polished surface. Remember, these coins were never minted for general circulation and should never be handled without protection.
Thanks for taking the time to read my guide. I hope I've been of help to you.
Good luck on all you bid for and I wish you all good health and prosperity for the future.
All the best, Ste.