Am assuming its not honed or polished slate here :
Slate doesnt absorb from above as its not formed the same way as other stone. If you grout it as normal using a sand based grout, allow the grout to just dry on the surface of the
tile then wipe off. Repeat the wiping process a couple of times keeping your sponge clean
In a couple of days when fully dry buff off the dust and apply a coat of Lithofin slate seal. This particular sealer will leave a sheen on the tiles, the sheen can be increased the more coats you apply. The beauty IMHO of this sealer is if you have missed any grout or have clouding it darkens it up so it can become unnoticable.
tiler
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