Porous Aluminum such as plenums, intake manifolds, trumpet bases

Some safety notes: You must wear safety glasses and a dust mask (aluminum dust is bad for you). I also recommend wearing ear plugs because you will be using the drill for long periods of time to sand down the metal.

What You Need

1. 40, 80, 180, 220, 320, 400 and 600 grit sandpaper
2. 40, 80 & 180 were 5 inch power drill disks
3.a drill
4. a mandrel for mounting polishing disks on the drill (available at hardware stores)
5. two 6 inch polishing wheels 
6. a Dremmel tool or high speed grinder to get at the hard-to-reach places conical shaped sandpaper drums
7. 3/4 inch cylindrical sanding drums (ACE, course, med, fine $8) conical shaped felt bobs for the Dremmel tool
8. tripoli and white metal polishing rouge
9. clean rags

One can try Bright Works at their web site: www1.minn.net/~bright-1 or (612) 429-4439 in the US. They sell a good book and video on metal polishing.

Sanding

I started by using 40 grit (course) sandpaper on a 5 inch wheel in my drill. This will take off the rough sand cast finish and remove the casting ridges. I used a small 40 grit, 3/4 inch sandpaper drum to get to areas that I couldn't reach with the 5 inch disk.

I then moved up to 80 grit (medium) sandpaper and went over the entire surface again. Do the same with the 180 (fine) disk. If you encounter an imperfection that you can't remove with your current grit, go back to a heavier (lower number) grit sandpaper and remove the blemish. I used some conical sandpaper drums to get to the hard to reach areas.

Next I used sandpaper sheets and hand smoothed the surface by wrapping the sandpaper around a foam sanding block. You will need to step down from 120, 180, 240, 320, 420, and finally 600 to get the metal smooth enough for final polishing. This is time consuming. It took me about 5 hours for all of the sanding (drill and hand).

Polishing

Polishing the metal was easier than I expected. The key is to make sure you have a consistent 600 grit finish on your aluminum part before you start. Any scratches or imperfections visible will still be visible after polishing, it will just be a shiny imperfection.

I used two, 6 inch polishing wheels that I got from a hardware  store (Dico #26 for the tripoli, #36 for the white rouge). The back side of the Dico polishing wheel package has a list of suggested wheels for the two rouges. You must use two different wheels for the two rouges.

Keep the subject piece at room temperature or warmer. This is important, the rouge will goop up if the part gets cool. I used a propane heater in the garage to keep the parts warm. Gently put the stick of tripoli rouge against the rotating polishing wheel for a few seconds to transfer the rouge to the wheel. Begin polishing the manifold with the polishing wheel. Don't push so hard that the wheel slows down a lot. Be careful here, the wheel will grab unexpectedly and can propel the part or the drill across the room. The polishing wheel will also grab onto sharp edges, so go easy.

Mounting the polishing wheels on a bench grinder is an option that works well but there are times when the drill mounted wheels are easier to maneuver. Reapply the rouge every couple of minutes. Keep going over the part until you get a good shine. I used my Dremmel tool with a conical shaped felt bob to polish the hard-to-reach areas. You will need two separate bobs for the two polishing rouges.

When you have a good, chrome-like shine, clean the part with a cloth and switch wheels/bobs and use the white rouge to bring out a mirror-like shine. You have to clean the part thoroughly before you move to the white rouge.

When you use a polishing wheel after it hasn't been used for a few hours you must "comb out" the wheel by rubbing it against a wire brush. If you don't do this you won't be able to polish the part to a shine.

This is hard work but the manifold (and turbo inlet pipe, intercooler pipes, etc.) look great. It really changes the look of your engine compartment. Good luck.