I have also used these paints to tone and disguise dutchman repairs and they work admirably. (Is there a tale here about Germans overcoming Dutchmen?) I have also seen potassium silicate used in quarries in the south of France to harden the surfaces of the soft limestone they cut. You can just keep applying the stuff until the limestone, usually so soft you can mar it with your fingernail, becomes so tough that a rasp chatters across the surface like it was glass.