Any tips on how to smooth out crown preps? NERB coming up.

Hi everybody! I have my NERB coming up in 2 weeks (#3 all metal, #5 PFM and #9 ), and I've been having a hard time smoothing out my preps. I usually have small "ribs" on the margins. Plus, my axial walls and occlusal surfaces usually end up looking rough. Any tips to smooth these areas out? Do you use slow speed or high speed to smooth out the "rough areas"? Have you ever used white stone (I've never tried it)? Thanks.

Any tips on how to smooth out crown preps? NERB coming up.