I'd think etching of the metal with any acid based blue or rust remover would depend upon the concentration of the acid, time, and temperature. Rustoleum rust remover appears to contain phosphoric acid, which would account for the gray coloration and slight frosting effect. This product probably would have removed any remaining case colors if left on the surface long enough, or at a higher temperature. It would have been very easy to walk away and let it work longer, and never know there were remaining case colors under the blue. It would probably etch the surface if left on too long as well. But the success of this is very interesting, and makes me wonder if the results for salvaging old case colors hidden under rebluing might be even better with a different acid.
