We do not recommend polishing your brass before resizing it. The oxidation and soot act as a lubricant when sizing which will make it easier for you to resize your cases. Also, clean brass tends to gall or leave deposits behind, which will build up and scratch your cases. While the scratching and scoring doesn't look pretty, it will not effect how your reloads perform at all.
If you find that your cases are being scratched and scored, you are able to correct it. Take your die apart and clean it with any available solvent, we use Hoppe's #9. Looking inside the die, you might be able to find the source of scoring. Take some fine grit sand paper and a wooden dowel and polish off the deposit. Be sure to lube the cases before trying the die again, even if they are carbide. Sometimes you are able to flake off the brass deposits using a stiff wire brush as well.
