Wet sanding is a must when it comes to polishing .With wet sanding from 300 to 1200. (or if you want to take it even lighter) With dry sanding at those grits your sand paper will hold the dust from the manifold in the paper, and cause heavier scratching then the grit of the sandpaper.
So when you use wetsanding paper and water, the build up in your paper will turn to a liquid, which won't cause scratching, you will see more of the mirror image your looking for.
Use a Dremel and start with an 80 or 120grit sanding drum to strip down the texture on the maniflod first, then start with a 120 dry paper. Then work with 220dry, and then go with 320 wet, and work your way up.
That's the way I've done quite a bit of my own parts in the past.