I sealed the tank on the wifes Zephyr after the tank rusted inside enough to turn the fuel into an orange goo, that clogged her carbs. The sealer I used is called Kreem, and cost about £30 ($60) and its a three part treatment, which you need to bake for about 40 mins in a warm oven to cure the sealer.
Although it was a little bit of an embuggerance, with the cleaner treatment needing to dry off before, degreaser went it, it was pretty straight forwards, and since the kit contaned enough to do two tanks I've also done the RT.