I definitely agree with the "don't clean coins" mentality... But I will say that sometimes you can't avoid needing to. I have lots of WWII Era steel, zinc and aluminum coins that have rusted or corroded over the years. They are intrinsically worthless in that state and there isn't much in the way of solvents to clean aluminum or zinc oxides off coins. So... Out comes to jewelers brush. It's an ultra fine, oiled brass brush. It's so soft it feels like cotton. It's abrasive, and will definitely leave hairline, but it will remove the unstable surface of the coin and leave you with a reasonable looking piece. Never, and I repeat never do that with anything of value... Because that's how you remove value not just dirt. Mild solvents are the best... Olive oil, watered down soap, or more risky ones like watered down acetone or jewelers pickle.