Seeing how the actual hardening is only microns thick, what hardening the engraver cut though is gone. If you desire to have the action re-case color hardened, it will have to be annealed. To be honest...it should've been annealed before polishing and engraving.
If the case colors are not desired, brush the colors off with a fine bristle mild steel brush when the gun comes back from hardening, gun makers do this all the time, its not a big deal. You can use different techniques to achieve different results, like aging or "antiquing" the action or brushing it shiny or doing a "French gray".
Dustin
