What's wrong with using a file, or two files for that matter?...The first one a fairly rough or medium cut to remove the majority of the braze-on depending what, and where it is ..then a fine cut to do the careful finishing down to the frame tube...almost...followed by some good old stropping with silicon carbide or aluminium oxide papers to polish off the last traces of the brazing or solver-soldering rod.
I would not go within five yards of a good bike frame with a grinder...Think craftmanship !!
Norris Lockley
Settle UK