nhjen:

You need to get a slate scrap for practice. Call the guys at Braxton Bragg, explain your situation, and take their recommendations, please.

OK! Here is what has been done...

After talking with a man who runs a quarry and fabricates slate, I felt confident that I wouldn't totally screw the surface up by sanding it. I used my Makita ROS and a ton of sandpaper and worked on the smallest slab in my kitchen, figuring that if it was destroyed, I could replace it and it wouldn't be a big deal.

For reference, this piece is ~18"x24" and it took at least 4 hours to complete the work. By this process, I won't be done with my kitchen for a while. And by most standard, my kitchen is pretty small.

Things turned out fine. I set the orbital to the lowest possible speed (2800 rpm) and applied a 220 grit aluminum oxide sanding disk (5") to the hook/look backing. I moved slowly across the slab - I used at least 8 pieces of paper to get this first part done, then I increased to 320 grit then to 600. My 600 pads were silicon carbide. I further used 800 and 1000 grits, but I don't think that they made much of a difference. Up to 600 though, seemed good.

Machine marks are clearly gone. I did apply a sealer: MB21 color enhancer. I applied it sparingly, then removed excess within 5 minutes. Is the surface well sealed? I don't know. But, Vermont slate isn't supposed to "need" to be sealed. The color enhancer improves the look of the counter though.

I have a slate slab in my dining area that has not been touched (to my knowledge) post installation (other than daily use, of course, so it is scratched). The surface of the stone looks not significantly different than the surface of the stone I worked on. IMO, it looks good. I'm attaching a photo, though I'm not sure that it's very informative. It's hard to get a good photo of a countertop. So, onward to the other surfaces. Unless I find someone to hire in the interim. It's nice to have a solution.