The process of polishing aluminum is simply to make it smooth (as glass). Simply start with a medium that will remove the roughness (maybe 150 grit?) and then move to less and less abrasive material. In the end, before the polishing wheel, you need to be sanding (smoothing) with 1500 - 2000 grit paper. Of course, the challenge with a manifold is all the nooks and crannies. You'll have to get creative for some areas (dremel?) It probably will take a week...BUT, once it's done, it's done. Just the occasional touch up... Get after it!