Your better off taking it to a metal polisher, you'll spend a lot more on mops and compound than they'll charge to polish it. If you have access to a 4hp polisher then you can just sand it with 320grit and then hit it with a hard mop then finish it with a soft mop. Using a bench grinder (unless its a big one, 3 or 4hp) is a pain in the ass because they just don't have the torque needed to put pressure on it without stalling the grinder, thats why you'll need to sand to 2000grit or finer with a bench grinder.