It is best to use unsanded grout for slate floors.
How to grout slate floor tiles.
By sealing your slate tiles you are blocking the pores of the stone this will then enable the grout to float over the pin head sized holes on the stone not stick within the pores the grout should now be easily removed when it is applied you are now ready for grouting the slate tiles.
To avoid spreading the excess grout across your tiles use one side of the sponge to wipe along the joint.
Replace or reinstall any remaining damaged or lifted slate and let the floor dry overnight before grouting.
Begin in the corner farthest from the doorway and work backwards.
A tip for you.
2 spread the grout over a small joint.
Fill a plastic spray bottle or a small pump sprayer with grout and tile sealer and spray the slate tile evenly making sure to get the sealer on the inside edges of the tiles.
Scoop a small amount of grout on a grout squeegee.
Use a grout float to press the grout into the space between tiles.
Once you have done this only use the other side of the sponge and only once.
Scrape away any excess grout.
Then start to apply the grout into the lines and joints in between the tiles by pressing it downwards.
After both sides of your sponge have grout on them rinse your sponge off and repeat the process until all joints have an appropriate amount of grout.
Select an area 2 to 3 feet square to begin and apply the grout with a rubber faced trowel to spread the grout across the slates working it into the channels between the tiles.