Brownells has some pretty good stuff. their black stuff T4? or something like that, I cant think of the name I use all the time holds up well and if you can warm up item you are doing does even a better job. My dad had a rem nylon never fired that someone didnt taker care of and when I got it done and gave it back to him it looked bran new. I have used it on Esee letterings etc and it turned it super black quick, plus blade in only coat. It's my go to stuff for anything I want blued/blackened. There other versions of blue agents are good too but the one that they say matches all blacks is my fav. still guna need some oil or grease for total rust prevention but in house without anything else I havent had a problem.