On bare steel you can give it a hcl wipe down . This will quickly remove carbon, rust in varing degrees of severity. Just rinse with very cold water and dry quickly. A great source of weak hcl is toilet bowl cleaner get the watery stuff not the gel or thick versions snobowl is what I was taught on. We would clean all our tubes, lugs and fittings in it before brazing. Just do not leave parts in too long or they will get eaten by the acid. This was a hand me trick from one of my predecessors and the great grandfather of a few of us MN frame builders Cecil Behringer