Diamond plates are arguably the most important component of the hand sharpening process, marginally less critical than the sharpening stones themselves.
Diamond plates have two primary uses. The first is heavy grinding of blades and bevels, as the coarse diamond particles remove metal faster than waterstones. The second use is to flatten waterstones, as sharpening any tool or knife will dish a waterstone to a greater or lesser extent.
The first requirement we have of our diamond plates is that they have an aggressive, hard wearing cutting medium. We have found the Atoma plates to be excellent on both fronts. The #140, #400 and #600 grits are all coarse enough to flatten the bevels of woodworking tools.
For heavy regrinding of profiles or severely chipped blades, we recommend the #140 plate. However, it's extreme coarseness means it should be be used on fine high-carbon knife bevels only with a lot of care, or not at all.
The #1200 plate does a great job of refining the scratch patterns of the other three plates, and allows the user to step straight onto a #1000 grit waterstone as the next part of the sharpening process.
The second requirement of any diamond plate is that it's substrate needs to be flat. As we use our diamond plates to flatten all of our waterstones, using a plate that is not true or flat can have damaging effects at all stages of sharpening, as it pushes all other waterstones out of flat as well.
After finding irregularities in other brands, we have decided to stock Atoma diamond plates as we have found the their aluminium substrates to be the most consistently flat of any variety we have tried. Additionally, Atoma diamond plates can also be purchased as a single sheet, so the user can either replace a worn plate or choose their own substrate - more information on that here.
We recommend Atoma diamond plates as a hugely versatile tool that will improve your sharpening out of sight - both for having a known flat surface to form bevels on and flatten waterstones with, and for speeding up grinding and the maintenance of stones.