Although flattening provides some cleaning, it doesn’t cleanse whetstones entirely. Over time, metal flecks and mineral fillings will clog the whetstone’s pores, making them sharpen blades less efficiently. The easiest way to clean a whetstone is by coating the surface with a generous amount of WD-40 and scrubbing using steel wool. While scrubbing continues, all the accumulated dirt, metal flecks, and mineral fillings will resurface, leaving you with a clean whetstone. Read more on how to clean and maintain a whetstone.