I have some .625" diameter aluminum bar stock that I need to do some kind of
surface finish/treatment to.

The stock has fine longitudinal striations (I'm guessing from being
extruded) that I would like to eliminate or at least reduce. I don't need a
superfine or polished finish- I just want to get rid of the "unfinished"
look. A fine, brushed finish would be perfectly acceptable.

Now, here's the problem. I only have access to typical homeowner tools and
no way to secure the stock in a fixture other than my own hands. I have a
big vise but can't use it because the jaws will mar the stock. I do have a
good bench grinder but I think it'll be way too aggressive. I also have a
belt sander and a random orbit finishing sander that might work. What do you
think? And what type of abrasive /grits should I look for?

Thanks!