This is not for razors, but my father-in-law uses a two sided/two grit water stone to sharpen kitchen knifes, and he has worn a concave surface into it. (And, ah, yeah, he's not the best sharpener).

Now that I am trying my hand at sharpening/honing, I thought I would try to repair his stone and give it back to him.

I tried sanding it but I'd go through a lot of sand paper before getting it close to flat again. It is a cheap stone, relatively speaking, and one side--the side that is least flat, is a pretty low grit (guessing 300-500, but what do I know).

Anyway, I don't want to use an expensive shapton lapping plate on it. Am I being too paranoid?

Are there other options (like rub it on a rock?)