Rot and decay are the universal enemies of all wood. Fence posts are no exception. Given their constant exposure to soil-borne bacteria and moisture, protecting them with a specially formulated wood preservative can extend their useful life by many years. It is an easy task to accomplish, provided you take the time to do it right. Before dipping your posts, they will need to be dried thoroughly and prepped to accept the preservative.
Measure the post from the bottom, and mark the wood with the saw or marker one-third of the way up. As an example, for an eight-foot post, you'd make your mark 32 inches from the bottom. The lower one-third, the part that will be buried in the ground, is where the preservative needs to be applied.
Using your drill, make a series of shallow holes around the perimeter of the post. Drill several additional rows of holes at six-inch intervals toward the bottom of the post. This will hasten the drying of the wood, and allow the post dip to more readily penetrate the heartwood. Set the posts aside in a dry, warm place until they are thoroughly dried out before proceeding any further. Depending on how hot your drying area is, this process may take several weeks or months.
Open the five-gallon can of post preservative. Stand the bottoms of the posts in the can and let them remain there for several days. If the area to be buried extends above the top of the can, use your paintbrush to apply multiple coats of preservative to those sections.
Remove the posts from the dipping container, and lay them across the pair or pairs of saw horses until they have dried thoroughly.
Dig your holes and set your posts. The preservative you have applied should extend their useful life for many years.
If possible, try to procure black locust posts. Black locust is naturally rot resistant. If you can, cut, point, and drill your post stock in the early autumn. Let the posts dry all winter. When springtime rolls around, they should be thoroughly dried and ready to be dipped.