The center of a waterstone gets the most use no matter how careful you are to use the entire surface of the stone when sharpening. This means the stone is likely to get dished out in the center after just a few sharpening sessions. One of the quickest and most effective methods I’ve found for flattening the surface is to “scrub” the stone using a sanding screen.
Sanding screens can be found at most home centers. They are generally used for smoothing drywall joint compound. I used a 120-grit screen taped to a flat surface for my waterstones.
Before starting, draw a wavy pencil line on the bottom of the stone. This helps track your progress. Using a circular motion, rub the stone over the flat screen. If needed, sprinkle the screen with water occasionally to keep it from clogging up. When the pencil lines are gone, you’re done.