You can do this when you have your rear wheel apart to relace to a 16 or whatever ..

The center part of a 750 rear hub has rough casting marks amid fins that are thought to be necessary for cooling .. They may help some when your brakes get red hot.. But I've never found hem to be necessary.. Soooo .. I have a machinist chuck the bare hub up in a lathe and turn the center section smooth .. usually I have him leave the center point higher than the edges by the spokes .. Sort of a small pyramid looking setup.. That way when the hub is polished and plated it will reflect off the inner sides of the spokes and that edge.. .. but .. most importantly .. It's not just a boring flat surface there ..

There's no way a polisher can clean that inner area up in its stock form .. it always comes out rough ..

Once the hub is then plated and the rim and spokes are laced up .. it presents a much prettier package than the stock hub .