Using acids on aluminum is very tricky. I do a lot of weld repairs on aluminum pieces and have all the acids that are sold for this purpose and they are pretty useless. Nothing like Oxalic acid.
I have a large vibratory finisher that is basically a box of rocks that vibrates like crazy. Some of the rocks are pointy and some are round. It seems to "smooth out" surface variations. It can't do a full polish but it will smooth it out without removing more than a 'thou or so (.001")
The kids around here have me VF the valve covers on their "tuners" (small cars with bald tires) before they polish them.
We can give it a whirl if you like ?