In preparation for grouting, I cleaned and sealed the slate. Sealing the slate before grouting helps in the clean-up process and helps to keep the grout damp enough to bond to the slate. Because slate is a porous and absorbent stone the grout can become too dry because the slate will absorb too much moisture.
To apply the sealer I used a 6″ sponge roller. I rolled it onto the slate with a bit of pressure to ensure enough coverage, and then went back over it to pick up extra sealer and avoid any puddling.
After the sealer had sat for a few minutes I also used a brush to pick up any sealer still left behind and to work sealer into corners and cracks.
Following the directions on the bottle I allowed 30 minutes before applying a light second coat. And the end result is a beautiful sealed floor ready for grouting.