It's loaded with half walnut and half corn media since I didn't buy enough the 1st time. I've done about 3-4 thousand rounds already and this is my last batch which is .338. I've ran it multiple time for about 5hrs but the neck is still dirty. I've also tried the Nu finish polish with dryer sheets. Is my media too used up and dirty already? How do you tell when it's time to change media?

Wet tumble 4 clean brass   

I agree wet tumbler and never go back.  When I dry tumbler, walnut get very brown and I then toss it about 6 times.

While I can't tell exactly what it is on your necks I can help you a little bit. There is a difference between tarnish and dirt/crud. I suggest scratching it with your fingernail. If it comes off it is dirt/crud. If it doen't come off it is probably tarnished beyond the polishing/cleaning capabilities of your media. If it is dirt/crud it is beyond the capabilities of your media to clean it. It is dirt/crud I would suggest what was recommended above with the wet cleaning. If it is tarnished no need to tumble any further unless you want perfect clean brass.

With all that said I just read an article regarding the importance or lack thereof for cleaning beyond what corn cob can do. And in some instances cleaning may not even be necessary. Mind you I am not making recommendations here nor do I fully support this way of thinking. But the gist of the article was that the author had not done more than a tumble in corn cob ever for his rifle cases and semi auto pistol cases. And he has never tumble his revolver cases, ever. For 25+ years of reloading. He claims that the wet lubricants are more than enough to prevent dies from getting scratched. And there is no need to clean revolver cases ever. Plus he claims he has never cleaned a primer pocket in over 25 years. His theory is if a primer can launch a bullet down a barrel it is powerful enough to overcome dirt/crud in the primer hole.

Just some things to think about.

Agree with Inspector and that essay.

If that black discoloration bothers you, there are 2 methods I know to get rid of it.
1. Scrub with steel wool or 600 grit wet or dry or...
2. Chemical clean via wet tumble or otherwise. Usually with a mild acidic solution.

Personally, I have used bathroom cleaner and ZUD to attack bad tarnish. Then dry tumble for a high shine.
The active ingredient I look for is phosphoric acid. Maybe coke will work too.

Looks clean enough to me.  That crud on the neck won't effect accuacy or performance. 

If you want to go all ocd, when you've spin to trim neck, hit it with some 000 steelwool, then neverdull cotton, and wipe clean with a rag.  It'll become as bling bling as you want it. 

Could that be heat discoloration from annealing?

If that's Lupua 338 brass and you don't like it, you can sell it to me.

Dude u need to change the media for every tumble session. U cannot use dirty media for dirty brass.

Dude u need to change the media for every tumble session. U cannot use dirty media for dirty brass.

That's the dumbest f cken idea I've ever heard. Pull your head out of your ass dude.      You must spend a fortune on corn cob.  Give me the money instead, ill get your brass clean.

 I've use media till it was powder.  Nothing wrong with your media.

Of course u should use clean media all the time.  Why use dirty stuff.

Corn cob is too
Slow.  I only use walnut media

Corn cob is too
Slow.  I only use walnut media

You should change your walnut media after every use.   

Of course u should use clean media all the time.  Why use dirty stuff.

Soap and water?  Bwaahhahahahhha

I use it about 6-8 times and use it as fertilizer

I'm too cheap to throw it away after every use

I don't know about the rest of you, but I only use bullets once before I replace them with new ones. 

LoL.  After the range closes I go out in the field and look for all the reusable bullets.

You should change your walnut media after every use.   

Use the Thumler tumbler to clean your brass.  1,000,000% better than the dry stuff, you will never regret it!

Changing your walnut/corn cob media ever time?  Sounds like a recommendation from the folks who sell it.  I have used these media multiple dozen times until they are practically black from the crap they remove - no problem.

Soap and water?  Bwaahhahahahhha

Yep