The original finish is an anodized surface. if you follow Alan's preparation up to the point that you get the pattern (very fine lines) that you desire, then bring it to an anodizing shop. That should restore to OEM condition. Member DeQuincey has made a very nice write up on how he did it, setting a new bench mark for e9 restorations along the way. Use the search button for his threads and be amazed with the wealth of info flowing your way.
If you bring it to the anodiser, have a look around your e9 and check what else is made of aluminium/aluminum that you can do in the same pass. Usually these companies have a minimal order price, making it interesting to do a bunch in one go.
Erik.