Scratch's on my cases

I've never been able to fix that the few times it's come up in my sizing dies, because I don't have the patience to sit with 1200 grit water paper and a dowel stick till the problem goes away.

What causes it? My .357 Mag dies do this, the only permanent answer I know of is to use another die set. I am suspecting something embedded in the die wall, my GS (Gun Shack) also polished the die in the workshop, tested it and gave it back as fixed - not for long !

The only way that grit could get into the die is by way of dirty cases. Tumble before sizing.

How deep are the scratches?

My 38Spl and 357 cases looked scratched, but those turned out to be light drag marks. One gets a similar result from the course side of a dishwashing sponge.

The only way that grit could get into the die is by way of dirty cases. Tumble before sizing.
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I only recently started tumbling before every reloading, used to tumble only when necessary (which it turns out is not only when dirty to look at).

I used Iosso paste on a tight fitting bore mop. Put the case in the lathe and polish the inside for as long as you can without falling asleep from boredom. Then do it for another 10 minutes.

In my case the inside of the die started to rust. That allowed some of the brass to stick to the side of the die. The scratches start small, but gets progressively worse as more brass sticks to the affected areas.

I used Iosso paste on a tight fitting bore mop. Put the case in the lathe and polish the inside for as long as you can without falling asleep from boredom. Then do it for another 10 minutes.

In my case the inside of the die started to rust. That allowed some of the brass to stick to the side of the die. The scratches start small, but gets progressively worse as more brass sticks to the affected areas.

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THIS !!!!
This may be the answer I am looking for, been years of speculating.The part you mention regarding rust, and that it gets worse, that interests me.
The OP is known to me, and I know his dies and weapon were on storage for 15 - 18 years (he moved to England after the Navy national service), my dies are (357) are from a deceased estate.

Maybe, just maybe !!!