Any wood—even pressure-treated wood—will eventually dry out, crack, and turn gray if left exposed to the elements. To protect your investment and keep your deck looking its best, finish it with a clear water-repellant sealer, a stain, or paint. To ensure long life, all surfaces of decking boards should be treated before installation.
Pressure-treated wood used for structural components resists the effects of weathering. If it turns gray and displays some minor cracks, it won't detract from the beauty of your deck because it's usually hidden from view. Even so, it's a good idea to finish the ends of structural lumber with a water sealer. The chemicals used to produce pressure-treated lumber sometimes don't soak all the way to the centers of the boards. Treating the ends protects against moisture penetration and ensures the longest possible life for your deck.
A deck with two kinds of wood finishes is common and requires two maintenance schedules. In this case, the cedar decking should be cleaned and resealed once a year, while the painted railings need recoating about every three years.