There is a difference between patina and rust. I would use this stuff sparingly and take off the rust but not the patina.
As far as the copper is concerned, make sure that what is in the solution is not copper or brass - the acid with steel probably creates an electrolysis with copper or brass, making the copper ions to migrate onto the steel, a natural result with many acid solutions.
Finally, I would suggest polishing and then adding a patina to the "lack luster" surfaces.
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