Hey Alex, I agree with the Triumph parts, they can be a pain in the butt. We have done quite a few Primary covers off of 60's model Triumphs and it is hard to get a good scratch free surface.
bewsted: From my experience it could be one of many things or a combination of multiple things. Here's What I would do......If the part was cast and not polished to begin with I would start with 80 grit sand paper and work up to about 320. After the 320 I have had GREAT results with the 400 Greasless Compound. The 80 grit should be the step in which you smooth out all of the casting marks and pits. For every grit after that your goal should be to get out all of the scratches left from the previous sand paper. Sand, Sand, Sand. If you think you have sanded all of the previous scratches out, sand it all again because you probably didn't get them all out. The best method I have found is to switch direction with each grit of sand paper and compound. After the 400 greaseless move on to the Emory, Tripoli, then the White compounds. I have also found that on some aluminum pieces it is benificial to finish the job off with the "RED" Rouge on a loose wheel.
It could also be that you are just being too hard on your work. I found in the begining stages of my polishing career, what I thought was a bad polishing job, the customers were MORE than happy with. Regardless of how much you polish, you will never get out all scratches and hazieness, all you can do is hope to minimize it. That's why people choose chrome for a lot of finishes, because it is much harder than aluminum.
Remember, there something like 40,000 diferent types of aluminum most of which will probably require adjustments in your polishing process.
Sorry that was so long, hope it helps.
D.J.
www.CHCustom.com