This double-strength blueing solution is designed for blueing highly polished steel parts and hardened steels containing nickel and chrome alloys (except stainless steel). Super Blue Extra Strength Gun Blue provides the blackest blue finish. Use caution to confine the solution to the area to be reblued, as it will have an effect on the existing blue. Apply Super Blue cold using cotton swabs, synthetic brush or sponge. Made in the U.S.A. by Birchwood Casey.