Results from lapping will depend on how bad the original surface is. 150 is too course. Buy a multi-pack of sandpaper for paint finishing. You don't need to polish it, so I'd skip on the 1500 grit stuff. You'd probably do best to start with 220 and work your way up to 600.
Of course every little bit helps, so if you'd like to go to all the work of polishing it, go all the way up to 1500!
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