Clean the slate patio well. Rinse the slate with fresh water to remove dust, sand or surface debris. Use one cup of bleach dissolved in one gallon of water to scrub up and remove any moss, algae or mold on the slate. Hose off the stones with fresh water to rinse. Let the stones dry completely before sealing.
Pour the sealer into a paint pan and soak a long-handled paint roller in the sealant. Roll the sealer over the patio in overlapping strokes to ensure even coverage. Let the sealer penetrate the slate pavers for up to 10 minutes.
Wipe the surface of each slate paver dry with a clean, dry cloth. Work the cloth around any clefts in the slate to remove any pooled sealer. Do not leave any unwiped sealer on the slate after 10 minutes; this will result in a film on the stones.