Double dipping makes your nails longer lasting and more eco-friendly. When rust forms on a nail, the nail corrodes, making it more susceptible to breakage than a treated nail. If dipping your nails drastically slows the corrosion process, it follows that double dipping them will make them last even longer.
Of course, the longer your construction nails last, the less often they'll need to be replaced. Double-dipped nails are therefore friendlier to the environment, making them greener than untreated nails, which must be replaced and discarded (or hopefully recycled) more often.
Another major benefit to double-dipped nails is that the materials that are bonded with the nails will remain stain-free for longer. Untreated nails used in visible areas -- such as decks, stairs or windows -- will eventually rust, leaving unsightly brown and sometimes black stains that form from the corrosion. Double dipping keeps these stains at bay.
Aside from better resistance to rust and corrosion, there's another practical reason why double-dipped nails are better than untreated nails. Double-dipped nails actually form a better bond with the materials they're holding together than untreated nails do, making structures stronger and more durable. The reason for this is that the zinc coating is rougher than the smooth surface of an untreated nail. Coarse zinc particles cling to the surface of the nail, enabling the nail to get and maintain a better grip on the material being hammered. This adhesive quality makes double-dipped nails especially useful for the parts of the house that are most exposed to the elements, such as window frames, stairs, benches and decks.
For more information on nails and green construction in general, check out the links on the next page.