David Weaver
i have a video on youtube about using the hard idler on a cheap belt sander as a line. you pretty much move the stone around on the idler (where the paper can cut aggressively) until you can get it no flatter and then move on to paper or diamond hone or grit. It turns out that you can do a pretty good job, fast, and a $2 belt from harbor freight would do several of the most dished stones you could find.

a good mask is a must, or a serious outdoor breeze at least. Smells like fire, but silica is a known baddie, of course. Not sure if novaculite does the same, but no reason to test luck.

As far as cleaning the stones - the razor community (for reasons beyond me) was militant about not wanting to use oilstones, so they would soak washita stones and other old stones in simple green. If you're willing to wait a day or two and use simple green in pretty good concentration, it will cease beading of water on an oilstone with very little physical work.

Boiling is also relatively common. The oil that's in these stones when they're caked is almost certainly something that shouldn't have been on stones in the first place. It works just as well to use a diamond hone once or twice then the stone is flat and use a non-drying oil and then the surface will be smart again.