Of all the ingredients a quality coating might contain, a water repellent is most important. You can clean a deck that's gray and has mildew, but if it has splits and cracks, it's much more difficult to protect. What will minimize that problem is a good repellent to stabilize the wood from moving through constant wet and dry cycles. UV protection is another must. Without it, wood fibers dry out and unravel, causing splits and allowing in moisture.
Pigmentation provides some protection, but specific UV blockers are a big help.
Most quality coatings contain them, as well as mildewcides, Deft's Pedersen said. When dealing with decks, it's a good idea to show as well as tell homeowners what will be happening. Pick a spot on the deck, clean it, and apply the finish. Let the customer see the before and after. Therefore, its initial moisture content may be too high to coat effectively.