Many AlOx compounds are actually designed to break down, with friability and toughness being engineered specs by which users can pick & choose the grit, based on desired end-use (friability info can be found on AlOx abrasives mfr's sites, for the raw AlOx grits available; there are likely thousands of variations). I mentioned the breaking down of some AlOx compounds, because I have noticed such apparent behavior in changing aggressiveness with polishing pastes like Simichrome (AlOx grit spec'd by it's maker at ~9µ). I've always noticed it starts very aggressively, but then slows in cutting speed with use and yielding higher polish on well-used strops. I've noticed my Simichrome strop needed much more frequent replenishment with fresh compound, because they'd lose effectiveness more rapidly; I'd notice my edges weren't finishing as crisply as when the compound was fresh. My white rouge strops, on the other hand, have kept working as well as when new, in spite of them very quickly becoming black with use; I'd assume that compound hasn't broken down at all. SiC has always been known to be relatively brittle by comparison to AlOx, which makes it break down pretty quickly (much narrower range of friability and toughness) and polish more with some wear.
But, that being said, with two mixed compounds that don't break down, the coarser is always going to dominate how fine the finish can get. There's no getting around it. I'm sure the mixed effect in the end will be different in some fashion; and if that fits for what the user is looking for, so be it. I still see no advantage in doing so with diamond in particular, at least nothing that can't already be done with two strops in sequence.
David
Having never worked with Simichrome or Mother's Mag polish, I cannot comment with any confidence on those materials, neither have I personally worked with multi-grit diamond compounds.
Not to be too much of a contrarian, but I can say of all the AlumOx, SiC, CrO materials I have worked with, I have yet to find one that broke down to any appreciable extent on a hardwood lap or strop. Not even SiC breaks down to any appreciable degree unless a great deal of force is applied relative to what one commonly uses on a strop. Unless held very firmly in place, SiC tends to glaze over rapidly into a dull, blocky shape rather than breaking down into smaller, sharp particles. I would question the ability of an abrasive to be of any use on hardened steel that could be broken down between steel and leather or denim unless at high speed on powered equipment.
Most of the manufactured friability of abrasives is relative to powered grinding operations or situations where the abrasive is held in a vitreous bond or well-bonded belt. Even materials that are more friable tend to fracture off very small pieces and continue cutting sharp and consistent for a length of time before losing effectiveness, they do not normally present as an increasingly fine abrasive unless ground between two very hard surfaces. Is more likely that materials which appear to grind to a higher polish (especially if they seem to struggle at the same time) after a bit of use are doing so due to glazing of the abrasive rather than break down. Indeed, the initial fix for abrasive glazing is to help it break down more readily by applying more force, more speed, or a less conformable contact surface - failing that one would use a "super" abrasive (CBN, diamond) or switch to more friable material.
One can apply some abrasive and a little oil if necessary to release an abrasive from the vehicle, to a piece of clear maple, red oak, scuffed aluminum even, and get a good idea of how friable it might be. Materials that break down readily on the surface of a vitreous stone and continue to cut sharp, do not appear to fracture down anywhere near as easily on softer backings at moderate amounts of force.
There are plenty of reasons why a preparation might lose its fiz, starting with glazing - the actual sharp corners of the material being rounded over. Another factor might be loading - the vehicle becoming swamped with swarf, or interfering with the abrasive after being pressed into a conformable surface.
Moving into the multi-grit strategy, I completely disagree with the assumption the larger particle size will dominate, though it will make its presence felt, and if properly managed will negatively limit the level of refinement to only a very small degree. Using Flexcut as an example, few individuals would note any real difference with a visual inspection or cut test between the final finish of the Flexcut compared to the final finish of a single grit compound at the smaller end of the Flexcut size distribution. What most or all
will notice is an immediate reduction in stock removal/recondition speed and ability to recondition greater amounts of edge wear, all other things being equal. It all comes down to nailing the distribution and proper vehicle and backing selection.
