Actually AA initially painted the A300s in the beginning but then reverted to the polished aluminum on the fuselage. Same as the rest of the fleet.
In fact, I believe that polished alloy aircraft require especially prepared skins, handled very carefully during the component and final assembly stages. If I am not mistaken, I believe AA needed their first A-300's quickly, before Airbus could make the required skin spec changes, and the first ten aircraft were delivered with semi polished Alclad, not quite to the fine polished skin standard of Boeing, or of Fokker on the F-100.