Wait a minute. It sounds as if your receiver is already shiny and virtually all the colors are gone. Why would you want to color case harden the buttplate and then remove virtually all of the colors you just paid for, and risk warpage, however slight, that would affect the fit to your wood? Is this an original Fox skeleton buttplate? If so, it is probably already hardened. The fact that the colors are gone does not mean the thin carburized case is gone. If you are not going to re-case color all of the parts that originally had case color, I would be inclined to just polish the buttplate and let it naturally discolor slightly as polished carbon sttel will do.
