I finally got around to sealing my tank with POR-15. Since the tank was new, the process was somewhat abbreviated but still took hours. It involved taping off the petcock hole and vent tube, and pouring Metal Ready (acid) into the tank. I used standard duct tape, which worked well.
Then I sealed the gas hole and left the tank in various positions over the course of a few hours.
Then I dumped the Metal Ready back into its container (it's reusable). The next step was to rinse out the tank with water. Since the whole point of this process is to prevent rust, it just felt wrong getting a raw metal tank wet. Then I used my heat gun to dry the inside of the tank. When it was completely dry, I sealed it off, poured in the POR-15 and then worked to ensure it covered the whole inside. Honestly, the toughest part was getting the excess POR-15 out of the tank through the petcock hole. Then I had to let it dry for a few days. I hope it works because I found this process to be a huge pain.