Brass in dies

[colonyfirearms]

I started to load some 357 sig and the cases were coming out with scratches
Inspected the sizing die and it had brass imbeded in the sides.
My son loaded some and I don't think he used any lub.
I took a knife and scrapped most of the brass out.
Used fine grade diamond lapping compound and shined the die. Sized another case. Measured it and I did not make the die any bigger.
But I still had a few fine scratches in the case.
Looked at the inside of the die with an eye loop and there was still some brass.
How can I get this remaining brass out of the sizing die???