Roofing tar is a traditional way to protect fence posts. Tar sticks to fence posts so nothing can penetrate, prolonging the life of the wood by keeping insects and rot away. This method still works today and although it is not attractive, treated fence posts do not need any maintenance. Treat your fence posts as soon as you receive them and they will last the life of the wood.
Now, when I place the posts, I put a collar around the post about six inches above and six inches below ground level. This can be paint or tar, I use tar for roofing as it is cheap and I usually have something on hand for repair work. I'm not trying to cover the bottom of the post since (in theory anyway) this will trap moisture in the post. It was a practice before pressure treating the wood to cover the part of the ground with tar to prevent it from rotting.
Roofing tar will take 12 to 24 hours to cure, depending on weather conditions such as humidity. Anyway, I just sealed my driveway with a tarry paint substance that looks pretty waterproof and I'm wondering if smearing some of that on the bottom and bottom would be a good way to seal the part of the post that's under the ground.