Whetstones wear unevenly in use. You can best notice the curvature of the surface by drawing a grid on the stone with a pencil (with vertical/horizontal lines). After that, grind the surface with a leveling stone until the entire grid disappears. Harder stones like the Shapton stones remain in good condition without leveling for significantly longer than others; however, they also require regular leveling.
Grit: #220
Long: 170 mm
Wide: 55 mm
Thick: 30 mm
Weight: 430 g