Stuff like that was still going on when I started collecting in 1960, but luckily for me my grandfather warned me against it. To a degree, stuff like that still is going on even today. And in some parts of the world it is far more common than it is in others.
Yeah, yeah, yeah. A youngster could buy numismatic cleaning kits from Sears in 1959. I know stuff. My Dad bought stuff. What stuff and to what degree and where in the world is it common?
Very true, when I was visiting coin shops in Mexico I was asked several times by different stores if I wanted them to polish the coins I bought. Lots of foreign dealers straight up clean their coins. It's saddening when you see coins that you know would be rare all polished up. But the coin cleaning taboo for a lot of countries is different from the US.