Steel “crud” removal

Any steel I buy has a dark layer of something. I’d like to make the surfaces to be clean raw steel for some projects. For years I’ve been using the grinder with a coarse wire wheel, but it’s time consuming and not all that efficient. I do sometimes use my belt sander and that method removes a lot of steel since most square tubing and other dimensional steel is not necessarily actually flat. Is there some chemical that cuts that crap. I tried metal prep and it removes some, but it is still slow. Lacquer thinner just takes off oil and dust. I’m too damned old to spend so much time cleaning steel surfaces. Ideas would be nice.
Jerry in Denver