Uncirculated Lincoln Wheat Cents - How Common Is "Coating"?

I have a steady turnover of wheat cents that come through, and I understand there was a short-lived trend of lacquering that is long dead.. but I know I do not often see coins that are coated, and the few I have seen were obvious to me. I had assumed the treated wheats are in the minority.

Could it be that there were many, many more that had been treated and most collectors/dealers could not tell? I am seeing a white waxy residue lodged in a couple of places on the legends on several coins I recently came by. If I had to guess, it looks like it could be a thin shellac that is not bonded to the surface. The areas are very small, so I am not yet sure I could capture it in photos since I have not yet photographed those coins. Other cents have surfaces that seem odd to me, such as this 1917..

A dealer had this labeled "MS63." I think it may have been cleaned, but mainly I suspect it has something coating it, mostly because of the "chipped" looking spot to the left of the date.



So mainly, my questions: Are there many of these that were treated? What could be the source of the wax-like crud on the uncirculated coins? Also, what could cause that spot on the 1917 cent pictured above? Thanks.