In terms of the cleanliness of the plates, I think you're worrying about nothing. Most people's plates are a bit grey, and not a few are tinged with rust from the same level of steel residue rusting sightly (reasonably common if the user is using a waterbased honing fluid or plain water). Neither seem to affect the way the plates work, it's primarily an aesthetic issue.
You can confirm whether it has any effect by carefully cleaning one half of any or all of your plates and then directly comparing cutting speed on the cleaned half versus the dingy half. My bet is that you won't be able to notice any difference.
Now that the main query you had is covered I wanted to touch on some of the incidentals.
The manufacturer recommends water for cleaning, which for me doesn't seem to do much:
Water and scrubbing with a brush using Comet or Ajax. If you're using only water that'll certainly account for some of it :-)
My problem with using water, is that whatever I use to remove the water from the stones seems to leave lint particles of cloth or towels behind, which then roll under my guide's roller or get in the way of the blade touching the stone. I've used towels, blue shop rags, J-Rags, and paper towels and they all seem to leave lint behind.
This has to be a common problem, I've certainly had it and I bet it's something that most have experienced. A few possible solutions:
- Dab only, no wiping.
- Blow dry using compressed air.
- Dry off using a heat gun or hairdryer.
- Just leave them to air dry (what I used to do when I cleaned plates under a running tap).
What I mostly do now is just brush off the swarf into a bin using a dustpan brush and stow the plates. Literally, that's it 99% of the time.
On the odd occasions I clean a plate down more thoroughly these days (and only on fine ones, never bother on my coarsest) I use a spirit-dampened wad of paper towel, wipe up with a clean paper towel and then brush off the worst of the inevitable lint with my hand. This doesn't remove everything but it removes enough that the plates work as they should.
I thought my planes were relatively clean, until I tried a simple pencil eraser, almost accidentally, and I realized how much swarf was stuck in them.
Bear in mind this residue went unnoticed because it's so fine. Even on the finest plates it could be composed of particles way smaller than the grit adhered to the surface, and as a result will have essentially zero impact on abrasion since it's just the peaks of the abrasive particles that do any work.
Only when the valleys between grit particles are completely full of swarf is cutting speed severely reduced.
The instructions that come with the plates mention that only a couple passes should be necessary for honing -- which does not match at all the performance I'm getting. It seems to take longer than it should.
I can't be sure but their instructions may be largely geared to knife sharpening, and few knives are as hard as even a half decent plane iron or chisel. A great many are significantly softer, most kitchen knives being a clear example. And the included angle at the edge may be lower as well (the edge is thinner). Combine the two factors and we're looking at two very different honing scenarios.
Anyway, I know they specifically say you should use light pressure, the quote you posted includes this advice. But IME it's perfectly fine to press harder1. Not hard (although I've done that) but certainly harder than I think they're suggesting2.
Pressing a little harder isn't just acceptable from what I've experienced using the diamond plates I have, it's the only way to get results in what I at least consider a reasonable amount of time — my goal being for day-to-day honing to take maybe three minutes and ideally less.
Obviously this means I'm not doing an excessive number of passes, but "a couple" is an exaggeration for me. Although five may be enough to raise a burr you can feel I tend to do about 10 back-and-forth passes (20 strokes total) to be sure I'm removing the entire of the wear bevel. For routine honing this is just on my finest plate. Diamonds cut fast so there's no need for anything coarser beforehand. I then strop and I'm done.
Note: if any of your tools are particularly hard or are made from a wear-resistant modern alloy like A2 you have to expect that all sharpening and honing jobs will take longer.
1 I've used diamond plates off and on for over 10 years and more recently (last couple of years) I've used them almost exclusively.
2 There's obviously some subjectivity in what constitutes reasonable and hard pressure.... just as there is with shaving pressure. I have always shaved using safety razors and I would presume this means I can press a lot harder than someone using a cut-throat can get away with!
Eraser on the plates. The cross shows before/after contrast.
