I have used about every cold bluing, and with a wide variety of results. Brownells Oxpho, is the easiest, and after oiling, does not have the tendency to rust. 44/40 is the strongest cold bluing, that I have ever used, however, it is very aggressive, and if not totally neutralized, it will rust, big time. You can steel wool and achieve a worn gray patina, but not a modern blue look. There are a lot of youtube videos on the subject. Larry Poterfield at Midway, has some good videos. I took some scrap metal and did a lot of testing. what I learned, is, you better keep an eye on anything you do, for several days. You might be surprised, to find it badly rusted. I'm not trying to scare you into not using it. But be aware of what can happen. Don't cold blue something, and stick it in a gun vault, and go back after several days. You'll know within hours.