Kieth I can't give you a scientific answer about what chemical process causes the various colors when steel is case hardened. I have however had good results bringing back rust blued barrels that have rusted as you describe simply by lightly carding off the patina with 0000 steel wool soaked in light machine oil. It might be worth a try and surely wont hurt anything.

Last edited by TwiceBarrel; 07/16/12 07:39 PM.