Klunk, I don't know if you have ever heard of Bath Power or Kitchen Power, it is a citrus based Australian cleaning product, not widely available in the UK, but it is very, very good at removing the oxide layer and especially limescale when cleaning stainless/ brass/ copper. I recently bought a box of it from Wilco as the major supermarkets no longer stock it they probably don't make enough profit on it, sellers are listed on Amazon. I use it a lot in my workshop on old motorcycle parts - it works.
Trevor