Diamond stones will sharpen the hardest blades. i can get a piece of dead soft mild steel sharp enough to shave with, but only for one swipe of 2. BTW hardness, wear resistance are not always the same thing.
If a hard blade, especially one of a steel with lots of carbides like D2 or M4 gets dull you are going to have to put some effort into getting it sharp again. A steel that forms carbides has tiny pebbles of super hard carbides that deflect wear. High wear resistance works coming and going. A straight high carbon blade like 1095 will get extremely sharp relatively easy, it will also loose that edge easier. There are no free rides
I don't care what you use, a whet stone, Arkansas stone, Diamonds etc The critical thing is maintaining a consistent angle. If your blade shaped object is very dull starting with a fine grit will take forever. Mostly I use a 2x72" belt grinder with a 2hp motor I can control the speed on. To sharpen I use an attachment that has an 8" open area between 2 2" wheels with the belt stretched tight. On stuff like the wife's kitchen knives and my hunting knives I use about a 600 grit belt at medium speed swipe on side then the other then dunk in water. I have 2 flex lights aimed at the work area. I keep a very acute angle on kitchen knives and hard work knives less acute. I keep checking the edge with the light and once I see a very tiny fine feather of steel all the way down the edge that goes from one side to the other as I make my passes, I am done with that stage. Next I go to a buffer with black jewelers rouge on it. I run thee edge back and forth until that feather of steel is completely gone. I end up with a very fine polished edge that is slightly convex so there is more support behind the edge than a total flat edge has. Not that that matters because my wife and step daughters use them on porcelain plates and a piece of glass counter cover. I can easily see the damage this does even to a real hard D2 blade. i generally use an ugly old butcher knife and a beat up looking paring knife that I am pretty sure are 1095. They stays razor sharp for a long time because the girls never use the ugly old knives.
If I get a blade that is beat up bad, I might start out using a platen backed 180 or 220 belt to reestablish the angle and get the edge down to reality.
Guys take a blade that is so dull that as my Grandpa used to say "you could ride it from here to Texas and never scratch yer butt", and go to swiping it on a hard 1600 grit stone and say it won't sharpen. It will if you just keep at it a year or so.
IF you don't allow your knife to become a knife shaped object it only takes a few swipes on a good stone or better yer diamond and a some passes on the back of a belt and your ready to shave.
I love GOOD diamonds "stones" They will not wear out, they stay flat. A scrub with soap and water and they are back in shape. I have a 2x8 that is medium on one side fine on the other. I have a hardwood block inlet to hold the "stone" with a thick piece of leather on the other side. My portable kit.
