M
MW Smith Ceramics
If I was doing it I would seal the grout as part of the process of..........
Clean the slate/seal the slate/grout the slate/.........and seal the slate and grout again.
The silicone bead???......for me it's.......leave the internal corners ungrouted,apply a silicone sealant(the same colour as your grout) in to the joint. The joints are best ungrouted so that the silicone gets into the joint and doesn't just sit on the surface of the grout.......well that's how I would do it.
I'm with wrighty on this one......exactly the same method as that!