Even if your deck is made of pressure-treated lumber, redwood, cedar, or some other durable species, it's at risk the moment the last nail is driven home. There are several culprits. Moisture swells the wood while the burning rays from the sun dry and shrink it, causing cracks and checks while also encouraging warping. Ultraviolet rays also discolor wood and accelerate wear by breaking down wood fibers. Add in the scratches that occur when you drag chairs across the surface, splattered grease from barbecues, mold growth in shady spots, and ground-in dirt from foot traffic, and most decks start to look old within a few years. Reversing the damage is simple enough. Apply a specially formulated cleaner and then treat the surface with a water-repellent finish. It's a systems approach where all the elements work together. Dirty decks won't accept a finish well; dirt can also react with the finish and turn the lumber black. And even the cleanest deck surface is at the mercy of the elements without the right finish. Choosing from among the several hundred deck cleaners, stains, and sealers out there is less simple. You'll also have to decide whether to do the job yourself or hire a pro. Here's how to take the guesswork out of renewing a worn deck or protecting the one you just built.