I do not have a lathe, so I have used a slotted jag with a section of cleaning rod in a drill with the wet and dry sandpaper mentioned below in a drill.
The same method with J&B Bore Paste or other polishing compounds on a “snug” fitting shotgun cleaning mop will also work well.
The copper solvent I used to remove the brass was Montana X-Treme Copper Killer. I sniffed it again tonight and it brought me to tears and cleared out my sinuous.
Tech Line & Tips (FAQs)
https://www.redding-reloading.com/tech-line-a-tips-faqs/149-scratched-cases-causes-and-cures
Cleaning Your Dies
We are often asked, "How should I clean my dies?" The answer is to use the same procedures, with the same equipment and solvents, that you use to clean the bore of your rifle or handgun. Use the same diligence and effort as well, since the brushing and copper solvents will work well on any brass deposits that may have accumulated inside the die.
Another question that comes up is, "Can I do any internal polishing?" Again, the answer is, "Yes, with care!" The best arrangement is to chuck the die in a lathe, after stripping it down and removing the internal parts.
Use #400 or #600 wet/dry auto body paper wrapped around a small wooden dowel. Keep the paper wet with a thin petroleum product with the consistency of WD-40 or kerosene. Using this method, you will easily be able to remove any foreign material or brass build-up from the inside of the die.
Since our dies are heat-treated to approximately 62 Rockwell C, you can polish for a long time without changing the internal dimensions. Do not use ordinary sandpaper or anything coarser than #400, as you will scratch the interior of the die surface and void the lifetime warranty.
It is very important to give the dies a thorough cleaning, after the polishing operation. They should be thoroughly washed, flushed, and wiped several times. Again, just as when cleaning your rifle, check the dies for cleanliness with several wet patches and then lube and dry patch them.
In summary, the name of the game when it comes to avoiding scratched cases is cleanliness! A little attention paid to keeping your reloading area clean, and periodically cleaning your dies, will insure that they give you years of trouble-free service.
If you have further questions, please feel free to contact our tech line (607) 753-3331