Hi Frans, I think we are both right here. Do NOT do this with a precious coin in beautiful condition.
I did this once, to a badly worn but very
rare early Iulia
Soaemias middle bronze (which
RIC Elagabal 401 calls an as) with
reverse MATER DEUM which cost me 5 Euros (and that's exactly what it looks like). The barely visible half-millimetre scratch will IMO not devaluate the coin, but allows me to call it a
dupondius. I did it for science. ;-)
Rupert