How do you seal a seeping battery terminal?

Well i used to make lots of batteries many years ago and the methods used then are possibly not suitable for the modern stuff. In the days of dynamos it was very common for this to happen due to overcharging,you just scraped out the crack to a V shape and using a small cintainer to pour in boiling black pitch,Felt washers 1.5 ins in diameter and quarter of an inch thick were set on the pillar, that is under the terminal think they were soaked in vaseline and it did work

There is NO safe way to seal this, it is defective, dangerous, and needs to be replaced. (Either it will likely cause a Hydrogen fire, which is high-temperature and hard to extinguish, or a build-up of Hydrogen will ignite, and give you a Hydrogen-EXPLOSION! Showering acid everywhere, and again, likely starting a fire. Add the likely major electrical damage caused to your wiring by the battery shorting out, and you would have to say, this battery is a lost-cause.

you cannot seal a crack or a defect you need to replace the battery especially if the plates have been uncovered the battery will fail in that cell anyway it has a defect. If its under warranty go get another battery .