Well this is one of my favorite sorts of questions.
So there are several sorts of approaches here. And a few controversies too.
So Ozuku Asagi tend to be quite hard. So one approach is to use a natural nagura (correct spelling). Classically these include stones like chu nagura, Tsushima naguras and the finer Botan naguras and finer Koma naguras. These are all coarser than the Ozuku Asagi stone itself, so the resultant finish is coarser than the Ozuku Asagi finish itself. You see this approach used a lot among straight razor users and it has classical roots among natural stone sharpeners going back to a popularization of this technique in the English literature, however, and this is somewhat controversial, the origins of this technique have roots dating back to what I would characterize as a 'geographical convenience' - these Nagura (at least Botan and Koma nagura) were available in a geographical proximity to stones like the Nakayama stones. So while it is a commonly used technique, my personal preference is NOT to use a fine stone as basically a platen for coarser particles, but to use a separate coarser stone as a stone preceding the finer stone. One of the restrictions using this technique is essentially grit contamination. This becomes a more serious concern with natural stones that are suita rather than tomae layer stones from the various mines, since the 'su' of suita layer stones are small holes in the stone. (There are some suita stones without 'su' but this is relatively uncommon). Using these types of nagura with suita stones cause the nagura to lodge in the su and mess up the finish obtained from the stone itself.
The more proper use of these nagura is not as small rubbing stones but rather as full sized stones used in part of a sequence for sword polishers. Alas, some of these full sized stones, such as Koma Nagura, have become exceedingly rare and expensive. I do have access to some small 'sheets' of Botan and Koma Naguras that can be used for this purpose. You will also hear reference to 'Asano' naguras, which are 'approved' as nagura among this group of naguras, however this is primarily a marketing issue. Again, you will hear opinions contrary to what I'm saying here on this subject, with references to a specific book about 50 years old on the subject that popularized this technique with a whole generation of barbers trained in its use.
So, moving on from this, let me explain another technique for slurry generation on fine hard stones - diamonds. This too has controversy worth exploring. So for the production of slurry you can use a fine diamond plate like the Atoma 1200 lapping plate. For this purpose, you can also use a small piece of Atoma lapping plate. I've sent Mark several of these in approximately 2x3" with a granite backing in All the grits - 140 400 600 and 1200. For the purposes of generation of mud on polishing stones, I would just recommend the 1200. The 600 and 400 work quite nicely for synthetic stones too. And the 140 works great as an edging tool for stones to round off corners of stones, etc etc. Very useful and at an affordable price. And quickly generates a coarser slurry.
So there are two issues here. Diamond contamination and particle size. It is argued that if a single diamond comes loose and contaminates the slurry it will result in scratches in your finish. Let's discuss this first. I would not recommend a brand new diamond plate for this reason, but rather break in your plate first. After breakin, the odds of loose diamonds in your slurry greatly lessen. I'll revist this again shortly. Second is the particle size of the slurry. Even at 1200 grit, the diamond gouges out grooves (much smaller than the 140 of course) in the stone producing a relatively coarse slurry when compared to the more slow release of stone from just prolonged sharpening. While this is adequate for some use, the resultant slurry is coarse relative to methods that produce a finer slurry (to be discussed). I would strongly recommend use of the 1200 Atoma 'slurry stone' as a means of texturizing the surface of a lapped fine natural stone that needed flattening as the surface is greatly improved vs leaving the deeper scratches in a hard polisher.
So another approach to slurry generation is fine diamond films
Diamond films are available as fine a 0.1 microns. It is exceedingly rare to find any natural stone much finer than 30k, with many polishers (awasedo) from 6 k or so. Again grit ratings on naturals is a messy topic beyond this current discussion. So generating a slurry with diamond films is certainly an attractive alternative and a dual use for the 1x6 sized films or even the full sized plates. This goes beyond the range of the traditional arguments about diamond lapping plates into a new realm. If a half micron or even 1 micron diamond particle falls into a 6k or 10k level slurry, will it affect the finish? Tenth micron - even less so. If a tenth micron particle falls into 50k natural stone slurry (pretty much an upper limit of natural stones) and lands in a 'su' or hole, won't it easily wash out, allowing suita stones to generate fine slurries without the issue of grit contamination being at all significant?
So now combining diamond films with natural stones suddenly becomes quite interesting. Using coarser films or plates we can create coarser slurries. And using finer films, we can create exceedingly fine pure slurries. We move beyond introducing another stone type (eg a traditional nagura) and producing mixed stone slurries and completely avoiding contamination issues - especially with suita stones. This is particularly interesting for straight razor use where less metal and slurry is produced because lighter strokes and less metal is being used compared to knife sharpening, so the harder stones traditionally used for straight razors can be extended to using suita stones too.
So next let me introduce another term - tomonagura. A tomonagura is a 'same stone' nagura. The stone you are sharpening or honing on is rubbed with a piece of the same stone to produce slurry. Ideally you can cut off a corner of the stone or a slice of it. Alternatively, you would use the same type of stone, being as precise as possible using a matching piece. Thus for an Ozuku Asagi, you would use a piece of Ozuku Asagi. For a Hakka tomae, another piece of Hakka Tomae (as opposed to a piece of Hakka suita). You do need to be careful to trim the 'scale' often seen on the backs of natural stones for this (another topic) to avoid bad results.
Tomonagura are particularly effective for very hard stones - like the Ozuku Asagi being discussed here. The resultant slurry is a pure slurry, suitable for the finest level of finish the stone is capable of producing. The particles are all 'the same'. There is no possibility of contamination from another stone type. For full sized stone users, consider getting the EdgePro sized stones for use as Tomonagura in a matching stone. A wonderful alternative use even if you don't use an EP. The Aluminum backing both supports the stone and allows full use of it down to the last bit. I can also cut smaller tomonagura on a custom basis from stones I have or alternatively cut off a slice from a stone of your own. Just contact me.
As an aside, I also use tomonagura on softer naturals, but for a slightly different reason. Speed and using up smaller pieces. You can generate a slurry more quickly with a tomonagura than without one. And half the slurry comes from the small piece so you use up less of the full sized stone to generate the slurry. For smaller pieces you can also use these like finger stones - a topic for another day. I will mention that I will be bringing in Hakka Tomae finger stone sized pieces on the next stone order. Hakka Tomae is a personal favorite stone of mine. It is getting increasingly rare to get and is particularly valuable for use on single bevel knives. Mark has a few of these in stock and if you are considering them, they won't be around forever.
So finally there is the new and controversial technique of using CBN and Polycrystaline diamond slurries as water based suspensions added to a natural stone slurry (this works with synthetics too). I broadly group this topic in two sections - using CBN or POLY of a similar grit to the stone and using a much finer grit. I'll only discuss the much finer grit option here, leaving the matching grit discussion for a later time. As a caution, use ONLY water based slurries or water soluble suspensions - not pastes, oil suspensions, or 'mystery preparations' as this can damage both synthetic AND natural stones! You can also use the water soluble Alumina suspensions for this (available in 1 micron, 0.3 microns and 0.05 microns) at CKTG. Avoid using CRO on stones! No, just don't do it! A mess from hell! While I tend to think of 'natural stones' as just Japanese naturals (my bias), this technique can be extended to Arkansas stones and 'Euro' stones, esp from Belgium. It works great on Arkansas stones. Michiel might want to comment on this technique for coticules.
So why mix in some CBN or poly into a natural stone slurry? Well especially if the CBN or Poly is considerably finer than the stone particles, two distinct effects are produced:
1) The stone slurry 'breaks down' more rapidly. Here the primary action is on the stone slurry rather than the knife / razor etc. Natural stone slurries break down (remember my bias toward Japanese naturals) with use. Adding a small amount (one spray or a single drop) accelerates this breakdown giving a faster rate of digestion of the slurry, producing a finer finish. The predominant finish is a natural stone finish. The CBN or Poly plays a background role here and you don't get a synthetic finish. With coarser CBN / Diamond, this isn't necessarily the case. Here you are purposely keeping the CBN/ Poly in the background. Also because the particles are significantly finer than the stone particles, they don't get 'stuck' in the stone or adversely affect the finish.
2) The CBN and poly are significantly harder than the natural stone particles. Even though quite fine, they EXTEND the range of usefulness of natural stones into being used on highly abrasion resistant steels. Because CBN and Diamond are both HARDER than say Vanadium carbide, you can now abrade the carbides too! Boron carbide is not quite this hard nor are aluminum oxide, silicon carbide and CRO. I wouldn't expect this second effect with the Alumina suspension for instance. Rather than simply eroding away the metal surrounding these carbides and relying on carbide fallout, you can now abrade the carbides themselves! I don't have the SEM data to back up this theory, but rather practical results and anecdotal evidence from other sharpeners using this technique. I've also discussed this subtopic with Devin Thomas, who also feels that this is a most reasonable theory, so I'm in good company on this.
So let's say you have a Ozuku Asagi and you are generating a slurry either by sharpening or honing a knife or razor on the stone. Generating a slurry is slow going. As you start generating the slurry the stone is quite hard and the mud initially unrefined so you are getting your coarsest finish. You can use a tomonagura to generate mud and refine it initially so you start off with a finer grit slurry. If you generated the slurry with an Atoma 1200, you will accelerate the breakdown to a more tomonagura level slurry more rapidly.
Now you add a DROP of CBN or Poly diamond (Poly abrades more rapidly than mono) to the slurry. The result is a finer more quickly generated slurry capable of being used on a wider range of steels - liquid nagura plus!
Well this turned out to be a very long post even for me. Hope I answered the question