Many coins fall into the FUBAH category, including ones with scratches or environmental damage; the most irritating ones are stained. These may seem like restoration candidates at PCGS or NGC. However, chances are they will be returned in their original ungraded-worthy condition because dipping — removing a thin layer of a coin’s surface — may be the only removal method, and even in those cases, the stain will remain, and the specimen will be permanently damaged.

I have written in the past about cleaning coins using MS-70 detergent or Jewel Luster (e-Z-est coin cleaner) dip. The former may remove light PVC damage or verdigris. The latter strips surface metal. True, some coins are candidates for a dip. Expert numismatists know which coins and what percentage of dip and water, etc., to use. (I can’t share formulas here.) But in almost every case, this devalues the coin, earning a details grade from major holdering companies.

Stained coins tempt bidders and buyers because they believe that the marks can be removed with MS-70. They can’t.

You can read the complete article on Coin Update which was written by Michael Bugeja.  It goes into much greater detail and has many example pictures.