Im in the process of polishing my water inlet pipes just to see how hard it will be to do the plenum. Its really not that bad, but I can't seem to get that mirror finish down. What do you plan on using after the rough sand 100 grit and wire brush? I could only find up to 120 grit sanding drums, then I started using a polishing bit and compound, but it still showed sanding marks. Guess I need to find some 220 grit drums somewhere.