I have worked out a fairly good procedure for polishing up aluminum. First try 600, if its not enough, try 400, and so on until you are removing all of the valleys and imperfections out of the surface. When I did mine I took a file and got rid of most of the weld marks around the wavy side of my valve cover so I went all the way down to 150 grit. From there It was 150, 220, 320, 400, 600, 800. when moving from one grit to the next sand 90 degrees from the prior grit, then you can see if you are actually removing the scratches left by the last grit.
Once you get up to 800 or so, it is time to break out the buffing wheels and use compounds. First get a sewn wheel and use emery compound on it, this should remove scratches from 600 to 800. After that wash the part!! and get a clean wheel and use tripoly on it, sewn or un-sewn it is up to you on the tripoly. This will remove the scratches left by the emery. Then wash the part. Then get a clean un-sewn wheel and use white rouge to finish it all up. This will leave you with a mirror shine. and it you took proper time and care to do all of the sanding right, the it will come out looking like chrome. Any questions?

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'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.