what i like to do when prepping stones for sharpening is take a pencil and mark the stone with an x pattern all over the entirety of the stone and then use a flattening stone until all the markings are gone, this will ensure you have a pretty flat stone free of and dishing or gulllying and it does a pretty good job of getting rid of any deep scoring that might have occurred during a previous sharpening session. you can look up stone fixer on amazon or google stone fixers. i would stay in the range between of between 100 grit to 400 grit. these grits usually take material away pretty fast so your stuck there all day trying to get a flat surface