I've been using a rounded 1/2 teaspoon of lemi shine and a cap full of car was soap and I was getting good results but I'd always have a few pieces of brass that would get these brown spots of tarnish. I might back both off from now on and see how it goes. I've got a real dirty batch of 500-ish 300blk to run after I re-run this Lapua brass so well see how it goes.
I wish I had a media tumbler because that was my first thought too
The formula I use to tumble my brass hulls is the following.
Too my Lyman Wet Tumbler Drum:
Add tumbling media (2 1/2 pounds of steel pins which have been previously cleaned, rinsed and air dried).
3 ea. 1/4 teaspoons of Dawn Dish washing Liquid.
1 ea. 1/4
level teaspoon of Lemishine.
Add brass cartridge cases (at least 50 rounds or more of deprimed Starline brass).
Fill with hot water to just below the inner cover seal.
(this will thoroughly mix all ingredients together)
Place a little Dawn dish washing liquid on the rubber seal as well as the female threads on the inside of the outer cap cover. This will prevent any water leakage during the tumbling process by creating a good seal. It will also aid in the caps removal when the time comes without a struggle or the need for a strap wrench.
Hand tighten the outer cap cover and place on motorized base
Run for about 1 1/2 - 2 hours
Remove brass and rinse in hot water
Place brass on a towel and quick dry (this will remove those ugly water spots)
Place clean brass in a case dryer and run @ 104 degrees for one hour. This will insure that all excess moisture from the inside as well as the outside of brass has been eliminated.
Hand examine brass and wipe any remaining residue off(and believe me there is always a little left on the brass itself)
Finally weigh and sort brass accordingly.
With this formula my brass is always come out looking like virgin brass.
Using to much soap or Lemishine resulted in a dull lackluster brass, however if you use this formula and you will get good results with the noted waters PH Level.
OBTW this is based on the use of PH 12 hardness of water.
To this end you may need to do some trial and error varying the quantity of either the Dawn dish washing soap or Lemishine.