I've done a C&L plenum 90%, came out pretty nice, had the local guy finish it since he was polishing my POSI Raceport blower. I hand ported the inside, got that where I wanted it, let him do the final buff out.
I just finished my idler bridge. There's a thread on this somewhere LOL. You need good power tools which different wheels. I have two drill presses in my shop, one bench mounted one on the floor. I can mount up two different wheels, and change speeds on both. The bench unit is an old monster, probably weighs 125-150 pounds, so you can push it hard just like my floor unit. I like drill presses because you can really move the piece around more so than a bench grinder with a buffing wheel. I also use a Foredom Tool, I have Chuck and Collet quick change heads for that. Good for smaller areas, great for porting.
Wet sandpaper is needed, I have a variety from 400-5000. It makes a difference. The bridge started out with a sandcast finish, many hours later, it looked incredible. Be prepared to get dirty, wear an apron and face protection, have plenty of nitrile gloves and Mothers Billet Cream.