A sharpening stone needs to be flat to sharpen well, but over time it will wear out from use. Sharpening stones usually start wearing from the center of the stone out, which eventually gives them a "scooped out" appearance in the middle. Once this happens to your stone, you must resurface it before you can again use it to successfully sharpen knives or tools with long edges. The fastest way to resurface your stone, if you can afford it, is by rubbing it against a coarse diamond bench stone. The below method takes longer, but it is less expensive.
Things You'll Need
2 tbsp. silicon carbide grit
Place a piece of plate glass on a sturdy surface where you can work comfortably. If you have some Mylar plastic available, wrap it around the glass to help resurface your stone faster. The Mylar works by keeping the loose grit from moving around the glass.
Sprinkle the silicon carbide grit on the glass or Mylar, then dampen the grit with water.
Put your sharpening stone on the glass. Rub the stone over the glass or Mylar surface until the concave section in the middle of the stone has disappeared.
Rinse the stone with water to remove any stray grit, then rub your fingers lightly over the stone to make sure you haven't missed any particles.
You can also use sheets of coarse silicon carbide sandpaper in place of the loose grit, but the stone will wear out the paper rather quickly.