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.