What kind of whetstone for sharpening a nylon edge?
I'd use fairly fine wet/dry paper taped to a hard  flat surface, probably used wet. You could move to a finer grade to finish. This will keep it nice and flat. 

Or you can shave/whittle the edge with a scalpel or razor blade. That's what I've done with a nylon spatula that I want a nice edge on, but on that the edge tends to deform rather than wear down, because it's used to push against hot things.