Need advice using vibratory tumbler and cast aluminum

[superjfly]

I am trying to tumble some cast aluminum pieces and the best finish I can get is a clean satin finish.

Here is my process:

Using 1/4" plastic triangles, water and detergent, for 24hrs. (This works well to clean and get things to a dull satin finish).

From that I go to corn cob with a tablespoon of auto polish added. Run it for 24hrs.

I would like to not have to finish the parts on a wheel. I am considering using hardwood cubes as a mid step or treated walnut. Any advice would be great.

Thanks,
Neil