Many years ago, when I had a local dealer to go and chat to over the lunch break, I recall talking to him about cleaning coins and he told me about a seller who knew nothing about coins, but had brought in a very nice and expensive collection of early milled in good grades. The dealer agreed a price and the punter brought the coins back a day or so later to seal the deal. Unfortunately, in the meantime, the seller thought that the dealer would like the coins to look a bit better, so he cleaned them up with silver polish so they were 'nice and bright and shiney'. Much to his horror the dealer told him he didn't want them, but was prepared to offer 10% of the agreed price for them. Exit one very crestfallen seller. Moral - don't do anything to a coin unless you know what you are doing, and certainly not before selling them!