In my experience the brass comes from not chamfering the neck O.D. Little pieces break off and get dragged down the die. I cleaned mine up with Butches Bore Shine, it contains a lot of ammonia that attacks cooper. I finished by cutting a slot in a wooden dowel and putting a narrow strip of 600 grit sandpaper in the grove and spinning it in the die with an electric drill. You don't have to get all of it out.
+1 but will add that this is done in conjunction with something like Kroil or kerosene to act like a surfactant and help lift those deposits with the abrasion.
I started reloading in 1970. At that time I probably used a Lyman product. About 15 years ago I was using a non-additized light vis synthetic oil I swiped from work. For the last about 8 years I have been usng the below product. It has light viscosity. It's about $9 a qt. A qt. could last 10 years. Never had any lube problems. I Always lube the neck since it's the most heavily sized part of the case. I always rolled cases on a pad. This oil wipes off in seconds with a paper towel.
A 20-50 oil may be more viscous and harder to wipe off after sizing.
10-4. I use corn or walnut to remove lube on 1000s of PD brass so it may still be worth the exercise.
I don't think Scotchbrite pads have an aggressive abrasive. I hand clean silverware with ScotchBrite all the time. It doesn't even scratch silverware. Wet grade sandpaper definitly works.
The typical 'green' pads won't do much. The 'red' one's such as I showed in the image are a different material.
