stole this from razzaken on here. i searched and pulled this up. i hope it helps:
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No matter how smooth the piece of aluminum I'm working is, I always start with 80 grit. Sand it w/ 80 grit until you think you've removed all the high spots, then hit it again for another good bit. IMO, the first round of sanding is the most crucial. If you miss something in the early stages, you won't get rid of it in the latter stages.
After the 80 grit, hit it with 220, 400, 800, 1000 & then 1500. Once you've gone over it with the 800, you should be able to start seeing a luster coming through. If you see something that you missed, drop down 2 grits (ie. If you're using 1000, go back to 400) & hit it again. Don't be afraid to sand, sand the shit out it.
Get a good & powerful bench grinder & put a cotton buffing wheel on each side. One will work, but two is the ticket.
Put a small amount on the part you want to buff, keeping the working area to about a 6-12 sq inch area. Doing a larger area will cause more problems than it's worth. Anyway buff the area nice & slow. Also, use light pressure, don't push the piece into the buffer. You'll more than likely cake your paste while buffing which will cause your buffer to 'grab' your piece & it will sort of bounce. If so, back the pressure off a little until your buffing nice & smooth.
Once your done with the small area, take a clean rag & remove the paste. For the areas that the paste caked, take some clean paste & rub over it. That will release the cake. A spray wax will do the job a little better. Also, you'll see the areas that the paste caked, they'll have a yellow tint to them as if it was heated a little too much (look close at my pictures & look at the runners, you'll see what I'm talking about). For those areas, take some 1500 grit & sand the color out. It won't take much effort. Remember, get everything off before buffing the area again. Clean between each buffing, and keep your hands as clean as possible too.
You shouldn't have to buff the piece more than three times, but keep going to suit your preference.
Get a palm sander, and a dremel will help immensely too. I have about 4 hours in my manifold, with most of the time being spent on the runners.
My advice is to spend more time sanding with the rougher grits, definitely not less. After skipping the lower grits & sanding for hours, I had to go back to the lower grits a basically start again because the higher grits couldn't take the larger cast marks off.
