I do the same minus the paint thinner!Tablespoon Nu Finish, tablespoon of paint thenner....run 5-10 min w/out brass....cut used dryer sheet into 4ths, add brass and sheets. I run 1-2 hrs. Throw away dryer sheets.
Have you tried just using plain tumbling media? I started tumbling my brass way pre-web and got no "internet wisdom" on what is "necessary", and walnut lizard litter (dark brown) from PetSmart worked great by itself...
I got a Frankfort Armory tumbler or father's day and tumbled brass for the first time with it the following week. I used a 5lb bag of white rice, a dab of Mothers Aluminum wheel polish mixed up into a bit of paint thinner. Very shiny smooth after a couple hours.