The thing here is to insure that the pits you are finding are going away in a wet or two. After the pits are gone, you can go to the next size of grit. What is happening is that there have been some very deep pits made in previous stages of grinding and haven't been removed by the next stage of grinding. I usually do a good check at the 320 grit stage to find the pits from 80 grit and, if necessary, go back up a grit to get rid of them.