Minor touch up to a flat blued or black oxided finish can be done with one of Caswell Plating's black oxide products.

I've had great results getting stuff nitrided by H&M Metal processing. As I recall it was $100 to run a slide through the Isonite QPQ treatment process. But that was a while back.

BTW... Isonite, melonite, tenifer, nitride, et. al. Are all basically the same and not a coating but a form or case hardening. The black finish is merely a side effect. It's not something that is applied but is a result of the part being submerged in a bath of molten cyanide salts though which nitrogen is bubbled. Even on old treated parts where the black color has worn off a bit the case hardened layer of the steel surface remains

Cheers
Bill

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