While we are on tumbling and surface finishes... One of the best things you can do is start with a good finish so that all of the processing steps that follow only improve on a good thing. Yeah, probably didn't need to state that for this group here, it kind of falls into the 'well ... duh' bin. Anywho, thought I'd share, one of those great steps for aluminium that I almost never remember until I'm actually at the machine is to pick up a piece of scotch brite.
Scotch brite does works well, quick swipe or four and a lot of the minor tool marks are gone. And yes it needs to be stressed that the end product is only as good as what is started with. A piece with tool marks to start will be a piece with nice and shiny tool marks at end.
So far at least for me
Shot: Preferably Stainless, works well to debur. This operation of course can be done with a file, sandpaper, yet for small pieces its hard to keep it from becoming too rounded over. Couple hours in a dry tumble with shot and the edges are crisp and clean yet you'd have to work hard to cut something with it.
Grit: Forget it, although it will remove minor tool marks and leave a semi finish (500 grit) it is just to darn messy for repeatable. Its bad enough to have that chip trail following me around, no need to compound it with black grit.
Media and Part load: Follow the chosen media's manufactures suggestion, at least start with it. Banking on their testing history which developed the process.
Media tool mark removal: unknown for me at this time
Media Finish: Limited only by the depth of ones pocket. I for one am going the walnut shell with compound mix. From what I have seen it should do the job for my usage.