What tool are you using? If you have it sanded down fine and lookin good, you should step to a rouge type product (comes in sticks like butter, different colors for different grits. And a metal polishing wheel on a bench grinder. I've never polished a throttle body, mostly stainless. Soi don't know how much shine is possible on that casting. But the method you're describing sounds like you're totally ready for polishing just don't have the right materials. Have you surfed up any kits sold for this? Search for metal polishing kits and you'll see.
