Spray the deck surface with a garden hose to remove leaves, twigs, and other debris. Nail down any loose nails. Sand any splintering or rough areas with a 220 grit sandpaper until the spot is smooth.
Mix 2/3 cup TSP cleaner in a bucket with 1 gallon of water. Scrub the deck surface with a nylon scrub brush. Rinse with the garden hose and let the deck air dry for 24 hours.
Pour the slip resistant deck coating into a paint tray. Coat a paint roller in the liquid. Apply to coating to the deck with the roller, starting at one end of the deck and following the grain of the wood. Apply it to the lengths of the planks, as opposed to across the planks. Strokes should overlap.
Let the coating dry according to the directions on the package before using the deck again.