Before sealing your deck, you have to ensure that it is clean. Remove any dirt in between the boards, then sweep the deck. Clean using a deck cleaner. I prefer applying the deck sealer on a cloudy day. After applying the cleaner, let it sit according to the manufacturer's instructions. Rinse thoroughly and let your deck dry for about 48 hours. Sand the deck and sweep it. Apply painters' tape to surfaces that do not require sealing. Use a natural bristle brush to apply the sealer: brushing forces the sealer into the grain and open pores. Ensure the sealer gets into joints.