Not all coin dips are the same, and some are far more friendly to coins' surfaces than others. Yes, almost every commercial product dip I've seen is harsh. But there are such things as gentler dips. The reaction is far, far slower with them. This was one main topic of my Saturday 9AM Money Talk at Chicago ANA 2014. The formula I use has a much more benign acid component. All dips use thiourea, a chemical that can both remove silver sulfides from a coin's surface and leave silver behind or it can be used to tone the same coin!!! The critical difference is the pH of the thiourea-containing solution. If it is acidic, sulfides are removed. If it is alkaline, sulfides are deposited. Nearly any acid or base can be used to modify the pH. Some are more corrosive than others. There are some really benign acids out there that work just fine - but you'll seldom find them in a commercial dip. I have used my formula on proof and BU coins that were graded (high) by both PCGS and NGC.