Coin might well have more value left alone than whizzed.
At a certain point, it is what it is, and any attempt to spiff it up is going to be very obvious and greatly detract from its value, whereas if its left alone it is in an authentic ground dug state, and no one is trying to deceive anyone.
[In most cases a soak in soap water and gentle tooth brushing to remove crud is acceptable] Soaking for days or weeks in olive oil is another way to remove crud without going overboard.
Much of what occurs to metal in the ground for long periods is a chemical reaction and cannot be undone [any more than you can undo the rust on your car door], attempts to do so damage the metal and therefore should not be done in a vain attempt to make it look pretty.