I'm about to buy my second tumbler - and it has to be the most used tool in the shed (garage in my case). I have a re badged Berry - a "Graf" tumbler. I use "Flitz" media with corncob.

I've used Novus2, Novus3 and Brasso, but nothing is close to Flitz. You only need a small amount, and you only need to tumble for 4 hours, not 24 hours. Secret is to clean the dirt off metal parts before you begin.