so far as welding mild steel on armor ....it cracks off easily so the Ni Rod offer is a good one
So far as sand blasting ...I am against it unless extreme rust is the issue. All blasting removes original paint. I prefer to spot treat rust and paint over old paint so its still there.
Original paint on WWII vehicles is extremely rare. For my own tank I have only ever over painted so I have been able to go back and rub down to discover original markings , camo schemes and colours.
I am happily in the same position with my FGT #9 . Blast it off and its gone forever...no possibility of future research.