The worlds three top grading companies all offer a restoration service. PCGS call is "Restoration service', NGC call it "Conservation", ANACS call it Conservation and strongly deny its a restoration service.
They all claim that they remove harmful contaminate from coins to protect the surface. They all state they do not clean the coins they simply remove surface particles including glue, stains, hazing, dip, corrosion and PVC residue.
Here is a before and after picture of a US 5 cent shield nickel from NGC
I have been in the industry since I was 10 years old and after over 40 years still heed my grandfathers advice "Never clean a coin, you don't own it you are just a caretaker until it is sold onto another generation". I am not sure how I feel about this. All three companies will (Unlike Mickey Mouse graders like ICG or CGS) spot a cleaned coin, I find it hard to explain the difference between cleaning and conservation to my customers when I don't really understand it myself. Worst still, once they are conserved/restored they are then graded with zero mention of this. This does not seem right to me or am I just old fashioned?