We have a member of our church congregation who lives in a doublewide mobile home with a galvanized roof that is old and needs either replaced or coated. He wants to lay a new metal roof over the old roof, but this is too much weight and too hard to lay flat and true. I can send you a satellite image of the roof. About 50% of the galvanizing is gone now and rusty, but we don't see any thinning of the metal itself. I would hate to try and sandblast it for fear of damaging it. Vinegar or phosphoric acid will turn the rust to salt. There are other more expensive treatments that will further add a protective coating afterwards. Your thoughts or experience?