Diamond sharpening stones are becoming really popular because of their ability to cut fast. We tend not to recommend diamond sharpening stones/plates because a lot of damage can be done very quickly. Also diamonds are pretty sharp and at arato level they leave deep scratches in the blade that need to be polished out quite aggressively. Diamond stones can be used with and without lubricant.
To take off the fine scratches and burrs left by coarser stones, and to polish the surface, you can use stones starting at around 2000 grit. There is theoretically no upper limit, but stones above about 10000 grit achieve practically no measurable improvement in the edge. It is also interesting to note that above 8000 grit, there is no Japanese measurement standard. For stones labelled as having a finer grit, you simply have to take the manufacturer's word for it.
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Actually, the company contacted me to make sure I received the stones and that I was happy with them. Excellent service. The stones are great! I'm pretty much a novice so the clear instructions that came with the stones and the eBook instructions were wonderful. The eBook had quality pictures demonstrating the use of the stones and the written instructions were clear and to the point. Within minutes of opening the box I had the stones figured out and sharpening my AF survival knife, my pocket knife and a couple of my wife's kitchen knives...which included a very expensive chef's knife. Very satisfied with this product and highly recommend it.
Historically, there are three broad grades of Japanese sharpening stones: the ara-to, or "rough stone", the naka-to or "middle/medium stone" and the shiage-to or "finishing stone". There is a fourth type of stone, the nagura, which is not used directly. Rather, it is used to form a cutting slurry on the shiage-to, which is often too hard to create the necessary slurry. Converting these names to absolute grit size is difficult as the classes are broad and natural stones have no inherent "grit number". As an indication, ara-to is probably (using a non-Japanese system of grading grit size) 500–1000 grit. The naka-to is probably 3000–5000 grit and the shiage-to is likely 7000–10000 grit. Current synthetic grit values range from extremely coarse, such as 120 grit, through extremely fine, such as 30,000 grit (less than half a micrometer abrasive particle size).
In most cases, the sharper, the better. The sharpness of your edge is determined by the angle (the lower the angle, the sharper the edge) and how fine of a grit you choose for your final honing. Since you have already determined your angle many steps earlier, now you just need to know which grit you can stop at. This again depends on the use of the knife. In most cases, go all the way to the finest stone that you own as this will give you your best edge. The only exception would be a knife used to cut soft vegetables like tomatoes, as a slightly more coarse edge will provide more of a tooth pattern for easier cutting.
My Fujiwara is an exceptional cutter and a pure joy to use, as a result, the mediocre knives get a pretty good break and the dream knife gets to do the lions share of food preparation. So it gets duller faster. However, it goes without saying that superior steel will have improved edge retention but at the end of the day, knives get dull, they just do and I am very happy about that, I get to sharpen them over and over!
Your primary goal is just to improve the edge, make it a little bit sharper. Manage you’re expectations and don’t worry about getting the knife ready for eye surgery on day one. Ignore all those YouTube videos where you see folks performing miracles with their knives, forget that completely. You just want to make the knife a little less dull and by doing so you will achieve what most people will never even attempt. You want to get a taste of sharpening success and it can happen quickly. If you are reading this, or any other cool sharpening tips, you’re on the right path.
Now you may wonder, if a finer stone produces a better cutting edge why do I need a coarse stone at all? The answer is that finer stones work much more slowly than coarse ones. The amount of metal a fine stone removes with each pass is much less than what a coarse stone removes. If you have an edge with a good shape to it from a coarse stone, this is not a problem because the fine stone doesn’t have to remove much metal to improve the edge. If the edge is dull, it will take many times longer to reshape it with a fine stone than it would with a coarse one.