Caustic soda solution will bring it up nice and bright. I used to use it on my tanker on the ally platework, side crash bars, fuel tank and other stuff on the truck while I was getting cleaned out at the tank wash. But be warned-its not nice stuff to come into contact with. Use pvc gloves, eye protection and have plenty of water to hand. What it essentially does is chemically etch off the top few layers of molecules. Application is a bucket and sponge job, rinse with a garden hose. Time between application and rinse depends on strength of solution and condition of metal. Try with a small sample first.