[Gregory B]

Orbit sander with 320 Grit paper until smooth or clean. As sand paper wears down, don't throw away. Continue to use old paper until completely worn out. This works as a higher grit paper and cheaper. Cycle paper as needed to use up and get the smoothest surface.
I have a battery operated Ridged Brand multi tool. I take the sanding adaptor, triangle shape, and wrap a small terry towel around it. secure it with a rubber band and use Mothers to polish.
I cleaned the inside of the fins and spray painted and allowed to dry completely before any of the above steps. After the polishing, simple green to clean polish out of fins and a quick repolish.
Power tools are the way to go.