People, I don't know.
I've heard if you clean, you might get more money 'cause the coins look better for sale.
I've heard not to clean because it does hurt the value, and the coin sellers tell people this so they clean at the shop and pocket the value difference.
I've heard cleaning for display purposes is OK.
I went here to see how:
http://metaldetectingworld.com/clean...ion_coin.shtml
Otherwise, I'm just a guy selling coins
that's all. I know it's a tricky business - fascinating, yes, but you gotta be careful it's true.