The coarse 400 grit Naniwa Professional Stone is for grinding a new edge on your knife, and working out nicks, cracks, or other imperfections in old, used, and damaged knives. Use this grade only when you regular maintenance and finishing isn't doing the trick anymore as it's quite coarse and so will cut (remove metal) quite aggressively and quickly. 

Naniwa Professional Stones are magnesia bonded. Magnesia bonding provides greater density and consistency of finely graded abrasive particles. This gives the stone a fast and efficient sharpening capability. Magnesia bonded stones do not require soaking in water prior to use, a splash of water is enough

After use, rinse the whetstone and wipe off the residue. Store out of direct sunlight. Do NOT place in dishwasher or store in a cold place. Gentle drying and room temperature storage is recommended.