In most cases, decks need to be resealed every three years. You can perform a simple splash test to see if water beads up on the surface of the wood. If the water penetrates the wood, it's time to apply a new coat of sealer. Many homeowners overlook the need to seal chemically treated and composite woods. If your exterior deck or fence is fading, warping, or rotting, then it is already succumbing to the elements.