Oh just that it can hide damage and bad steel in pictures. If you dont know about it then you can overestimate condition, as I did with my "shave ready" Filarmonica 14 that had perforations all the way through the bevel and needed a lot of time in stones to be usable.Agreed that it is necessary to have some knowledge and experience to do competent vintage purchase.
What is the issue with evaporust or other chemically removed rust?