George Finally got my ear, so I ordered up a Spyderco ceramic bench stone to play around with. I ordered up a 302F, their 8in x 2in fine bench stone. I have heard that it is anywhere from 1800-4000g. Even the guys that work at the company who post on their forum are pretty unspecific on that topic.
It arrived yesterday and I promptly opened it up and plunked a chisel down on it. No, actually I checked it with a straight edge first. Ugh, it was well out of flat (a nice ~ shape). Might as well check it out before I raise heck with the vendor or Spyderco. It cuts nice, but like a diamond hone it has rough spots. It also feels like a roller coaster pushing a chisel lengthwise on the stone.
I have heard these things are darn hard to flatten. Several folks have posted their account of said event on blade and straight razor forums (email me for a link if you are curious, forum rule prohibit it being here in the thread). Those that have done so successfully have either used a lot of sandpaper, damaged a diamond hone, or worn down a diamond hone during the process.
I spoke to a rep at Spyderco this afternoon. His response to flattening at home was pretty much that it couldn't. If it isn't flat it needs to go back to the vendor or him. He was a good guy and really made clear that he would take care of me, but it really doesn't sound like they rework or flatten many of these stones. If it is out of spec then they replace it. I have heard mentioned the spec is .020", which is pretty bad for straight edge blades and tools used in woodworking.
Well, I got to looking at the stone this evening. I had some free time. So I laid out some 80g Norton 3X on my granite surface plate. I penciled in a grid on one side of the stone and set to work. About six strokes in the Norton 3X was toast. Wow, those guys weren't kidding about this ceramic being hard to flatten . Okay, time to pull out the big guns. Silicon Carbide loose grit, 80g lapidary. I just sprinkled that onto the 3X, using the sandpaper as my substrate. About 1/4tsp gets you about 2-3 minutes of grinding before it breaks down. After 15-20 minutes it is a good idea to swap the sandpaper, so it doesn't get out of flat (it will anyways, but not enough to be an issue).
I found it best to do this dry.
**SAFETY NOTE: wear a respirator, ceramic dust is not a good thing to deposit into your lungs. I use a 3M 7*** series.**
This prevents a big mess, allows the spray adhesive to keep the paper flat, and allows me to vacuum the swarf away.
Okay, so apply paper. Loose grit it a couple of times, vacuum, loose grit it again, vacuum.... Rinse and repeat. Inspect with straight edge and monitor progress. I also used feeler gauges to give me an indication of progress. I started with .009" hollow and a low end of about .020". It took a bit over an hour to wear down to my stopping point. I called it quits when most of the stone was flat, no light under the straight edge. The low end is only for the last 1/8" and is .002" low, I can live with that. The extra effort to get that last 1/8in would likely take another half hour and really doesn't do anything for me.
I tried to match the surface sheen of the other sides of the stone, so I finished up with 220g SiC grit on some emery sandpaper. I wouldn't recommend, but I didn't have any high grit 3X around.
I actually think that this is too polished. I may rough it up a bit with a diamond stone to hasten the cutting action. For now it cuts good, about 6-7000g waterstone equivalent but a faster cut. I am going to have to evaluate it a bit longer for more observations on how it works. I did want to put up that you can indeed flatten these without toasting a $70 diamond plate. I am out 5 sheets of 80g 3X, 1 sheet of fine emery, 2 tablespoons of 80g SiC grit, and 1/2tsp of 220g SiC grit. That's about $10.