Using plastic sheeting and tape, mask off surrounding walls, concrete, grass, shrubbery, fences, patios or other adjacent areas that are vulnerable or that you do not wish to seal.
Wearing goggles and rubber gloves, clean older decks with a deck cleanser and brightener, a hose and a power washer. For a new deck simply sweep off all dirt and debris. Allow a wet deck to dry for 48 hours.
Pour Thompson's Water Seal into a paint tray. Using a paint pad and a 4-inch paint brush, apply sealer to the deck. Work the pad or brush in the direction of the wood grain and be sure to coat all sides, including the underside of the wood, where possible. Apply two coats within a 24-hour period.
You can also use a high volume low pressure sprayer. This is particularly good for large decks with many small posts. When applying Thompson's Water Seal with the sprayer, make sure it does not create mist clouds of sealer. Apply two coats within a 24-hour period.