conditioning a glass call

[Rio Fan]

I've got a double glass call that I really like, but even after I condition it with 150 grit sandpaper, it's still fairly slick.  I'd greatly appreciate some tips on what I can do to help solve this problem.  Thanks!

[shootumindaface]

Stone, heavier grit sandpaper or the Screening..

[bowhunter84]

Stone, heavier grit sandpaper or the Screening..

[drum817]

I use a primos slick stick and on a new call I will work the stone in one direction for 10-15 mins.   Sometime that glass can be tuff to cut.