If you have a brand new deck, you are dealing with wood that is not completely dry. If you try to put a sealant on it now, it will not penetrate the surface properly. You need to wait at least a year before sealing a new deck. For the first year, apply a new wood treatment product to the deck. This will provide the necessary protection throughout the year as the wood dries and the fibers open up. If you have an older deck, you should reseal your deck every twelve to eighteen months.
Most sealant products work best in certain temperature ranges. Consult the product's directions for detailed information. However, with most products, you should not apply a sealant in the cold of winter or the heat of summer. The product will not penetrate the surface properly. You need mild and consistent temperatures such as during spring or fall in most climates.
Sealing a deck involves more than just applying the sealant product. You need to clean your deck first to remove dirt and to get down to solid wood. Between the cleaning and sealing, you need at least two to three days to allow the deck to dry out. Look for a period of five to six days where the weather forecast calls for no rain and mild, consistent temperatures. On the first day, you clean the deck. You wait at least two days for the deck to dry. On the fourth or fifth day, you apply the sealant product. The next day gives the sealant time to dry and cure before the rain comes.