You can use something like CLR or LimeAway bathroom cleaner, both are phosphoric acid based. The acid will dissolve the rust a also roughen up the steel a bit. Just brush it on and keep it wet (brush on more acid) for about 30 minutes then rinse off and let dry. It'll leave a light surface "rust" coating of iron phosphate. You can get the real metal etching compound and it'll also have a zinc component to it in addition to the acid.

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