Looking for advice on sealing slate (or other stone in a shower)

I am using salvaged slate blackboards for the walls of a tub/shower in a bathroom.  The slate will be attached to a backer board and then screwed into the studs. To keep water from migrating through the slate (which is 3/8" thick), I would like to seal it, and I would like the sealer to be as durable as possible to the sort of scrubbing that a shower wall might get.  Does anyone have any advice?