The AC compressor looks like it was pretty tough. How did you deal with all the small grooves and channels? I've polished a bunch of stuff on my car and friends cars, using the hand method. Hand sanding from 100 grit all the way up to 1000 in various steps and then hitting it with Mothers Mag & Aluminum Polish. It's a real PITA. Last time I did it to a rough cast piece, I think I got sick from all the chemicals and aluminum (didn't wear gloves).
^ since this piece had a large flat area, I was able to use an electric palm sander for the majority of the surface. The piece on the left is pretty much as-cast with lots of sand grain and surface pitting.
My notes for it say:
"50 grit, 150 grit, 220 grit, 400 grit wet, 800 grit wet, 1000 grit wet, 2000 grit wet, Mothers Mag & Aluminum Polish w/polishing wheel on corded electric drill (should have used rotary tool)"