Most of us aren't concerned with the chemicals that go into a quality exterior wood coating. If the wood beads up when wet, we're satisfied. However, paraffin and similar waxes which create this visual effect do little to protect the wood. Exterior coatings need other ingredients to give long-lasting protection. 
According to Mike McEnroe of The Flood Co., wax produces a nice effect but doesn't last long by itself. "It's a visual thing, only good as long as it's not scuffed off," he said. "In high-traffic areas, you're going to lose that quickly."
Some quality coatings have wax - to satisfy consumers' demand for beading - but they have other raw materials, too. Many of these ingredients, such as mildewcides, are expensive. Discount brands use lower-grade raw materials and in varying degrees. So while consumers might save a few dollars initially, they'll end up spending more in recoats.
Of all the ingredients a quality coating might contain, a water repellent is most important. "Here's why," said Chris Hatton of Wolman Deck Care Products. "If my deck is gray and has mildew, I can get it off with a deck cleaner. But if it has splits and cracks, I can't do anything. What will minimize that problem is a good repellent to stabilize the wood from moving through 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 additional UV blockers are a big help. Most quality coatings contain them as well as mildewcides.
Remember, you get what you pay for, said Fran Calpin of United Gilsonite Laboratories: "Consumers may save a few dollars using a bargain brand," he said, "but, in the end, it really isn't advantageous to do that because they have to use the product more frequently."
To buy a quality wood coating, visit your local independent paint retailer.
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