I used a small file to grind down the seams and any numbers, letters, et cetera.
I then used 220 grit+sanding block to remove the castings and further reshape any areas that need it.
I then use 320 grit, then 400 grit DRY (this kinda sucks cause the papers has to constantly be cleaned), 600 grit WET, then 1000 grit.
Afterwards, get a polishing compound and have at it.
I must stress that the first two stages of the process (filing+220 grit and 320 grit) are the most important. I did the plenum on a manifold over the summer, looked 100% better than it does stock, but no where near perfect. I will be getting a new stock manifold from Shaun and will be polishing up that for a 2nd attempt. This time i'm not stopping until it has a mirror finish
If you have a decent sized air compressor and die grinder. You can use 3M rolocs discs to remove the casting (course pad, then medium pad, then fine pad), then start going at it with 400 grit WET, et cetera.