I discovered a hairline crack in my plastic gas tank and I'd like to know whether anyone knows a way to permanently seal it.

 

well you would need to completely drain you gas tank.... let it sit for a little while to let all the residual gas to flow to the very bottom part of the tank (if the hairline crack is not on the bottom face of the tank that is) and then try to seal it with a plastic resin of some sort.... if there is gas in the hairline crack or around it, it will not seal correctly

 

I discovered that I really missed it while I had it tore down while fixing the alternator. Of course the weather turned to crap and I didn't have my go to vehicle.