Regardless of the artificial stones or natural ones, whetstones are usually categorized based on their grits:
- Rough stone: #200 — #600
- Medium stone: #800 — #3000
- Finishing stone: #4000 and up
There are a few other types of stones that you use with the whetstones:
- A fixer stone: flattens the whetstone
- A Nagura-toishi: rubs against finishing stones to create a slurry (aka 砥汁) to help with sharpening (this is why the natural stones from Kyoto are called 合砥 awase-do, “awase” meaning “combine”; Nagura is used in combination with natural stone to create the slurry before sharpening.)
The smaller the grit number, the coarse the stone is, and the faster it scrapes away the metal from the blade. Therefore, rough stones are mainly used for repairing edge chip; finishing stones are good for daily maintenance; when the sharp edge cannot be restored by finishing stones alone, use medium stones to reset the bevel.
If you are just getting started on sharpening knives, a medium stone is usually all you need to restore sharp edges. The upgrade path should look something like this: medium stone → fixer stone → rough stone → finishing stone.
As you sharpen knives on a whetstone, the surface of the stone gradually becomes concave. I cannot stress enough how crucial it is to use a fixer stone to flatten whetstones before sharpening. Let me show you a few crude illustrations (drawn with touchpad and paintbrush).
Simply put, you cannot achieve a sharp edge on a whetstone with a concave surface. A stone fixer can effectively flatten a whetstone; without one, you can use a lower grit stone to fix a higher grit stone.
Natural stones are expensive and highly spoken of by the aficionados. Some believes that natural stones can give blades mirror-like shine that artificial stones cannot. I do not have much info on this topic at this time but will update in later posts.