Hi
The problem is the different metals involved. the hubs are made of Electron ( a type of magnesium alloy) the brake drum and sprocket is steel and the rivets copper , holding the drum to the hub . the hub was coated in the releasing agent when it was cast which is the gold colouring , then the drum and hub assembled by rivetting together, the only way to restore the original is to dismantle all the components that make up the back wheel assy and have them individually treated .I have yet to find anyone who can duplicate the original finish,but please let me know when you do .