The answer to this question depends on what product you are using. In general, exterior wood sealants can be applied with a variety of painting tools, including brushes, rollers, pads, and sprayers. Most wood sealers dry rapidly. It is important to use an application method that you can perform quickly. Thoroughly clean your deck before applying sealant. Dirt, debris, peeling paint, and even raised hardware will inhibit the sealant from forming a uniform coating. If possible, use a high-powered pressure washer to deep clean your deck before sealing it. Otherwise, clean and inspect your deck to identify and remedy any problem areas.