Remove everything from the room with the slate flooring.
Clean the floor with trisodium phosphate and the scrub brush to remove any dirt, dust or debris. Scrub the floor and rinse it well with water. Allow the floor to dry completely.
Dip the roller into the masonry paint and apply to the floor. Roll the paint over the slate tile to give it an even appearance. Use the paintbrush to paint the grout between the tiles. You can use a different color on the grout than on the slate, if desired.
Allow the paint to dry for at least 24 hours. Apply a second coat of paint in the same manner as the first and allow it to dry for another 24 hours before walking on the floor.