What grits are the "coarse" "fine" and "extra fine" equivalent to?

Allene is correct re coarse, fine and extra fine. However, grit sizes can be confusing because of the different grading systems of the particles. Abrasives with the same grit number can actually have different particle sizes. It all depends upon the grading system. For example a 2,000 grit water stone would not have the same particle size as a 2000 grit wet dry paper. There is even a difference among the paper abrasives. The Europeans use the FEPA system. Their abrasive sizes are preceded by a P (e.g. P220). Americans use the CAMI system. CAMI just has the number (e.g. 220). Again identical numbers might have different size particles when measured in microns. Therefore, I always try to get a reading on the particle size in microns. A micron is 1/1,000th of a mm. DMT also makes Extra Extra Coarse which is a 120 mesh (thats 120 little openings per sq. in.) and particle size of 120 microns. Extra Coarse is 220 mesh and a particle size of 60 microns. The coarse grit is 45 microns. Fine is 25 microns. Extra Fine is 9 microns. There is also an Extra Extra Fine which is 8000 mesh that is a 3 micron particle size. > 50 microns is considered grinding material. 15 - 50 microns is honing material and less than 15 microns is considered polishing material. I hope this helps.

Many of the reviews mention using this to flatten wetstones. Can I just use this to sharpen plane irons directly?

Here's the "what for" on this item You absolutely can sharpen any metal on this stone, but you will be limited to the actual sharpness and bevel width. This stone is made with larger diamonds than most coarse stones. Because of this, as well as its length and width (and skid-free bottom), it can take nicked and out-of-true edges and get them back on track very quickly (use water only, no oils). It is a 2X coarse blade, and awesome doing what it is intended to do (this includes grinding down chisels, whittling tools, whetstones, iron, steel, etc). It could be used to put a decent edge on your blade, but it really is limited to coarse work needed to prepare it for the proper stone; I recommend soft stones from Arkansas, they make great whetstones for fine honing, but any quality soft stone is the next step. To make this long answer longer, the reason the DMT is mentioned and recommended to flatten whetstones is that, again, the large diamonds go to work quickly and grind down uneven, pocked, or dipped anomalies on the much softer soft stones which makes plumbing a straight edge on a fine blade much more effective. So, basically, you keep the DMT to grind the soft stone to level, use it to work the blade to rid it of abnormalities, and then let that rejuvenated hard and straight blade make woo with that fine Arkansas (or personal choice of) whetstone (water, spit, and oil are optional). Sorry for the long story, take care of the ones you love, thank a vet, prepare, prepare, prepare, and have a great September; especially those of you who have the privilege of experiencing the changing of the colors! Also, it's the time of year we wait for; muleys, white tail, wapiti, antelope, and turkey (plus whatever else ya'll get to hunt and stock in your territory). Prep them blades, clean and sleep with them guns, shake them cartridges to loosen up and distribute the powder, and keep field stones handy for quick edges for the cold, morning, field dressing...And thanks for the question!

is this 3 stones in one package or just one? because the picture is three stones.

You only get one but they show the 3 different grits. These aren't stones but rather diamond embedded steel plates.

How flat is it? Is it flat enough to true up a waterstone? To put it another way, can it be a smaller/cheaper version of the DMT lapping plate?

Its very flat I use the coarse and extra coarse to flatten water stones all the time. I would recommend them to anyone for just water stone flattening it works great but in also use it to sharpen with great results

Dia-Sharp Benchstone Diamond, Coarse Grit

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