There is a product called "Seal All" that works like magic on gas tank leaks. On a poly tank exposed to the elements I'm not sure I would trust it for a permanent fix, but it will stop a flowing leak as a way to get through a job. I've used it on metal tanks and is is fantastic, only used it once on a poly 5 gal can. It seemd to hold for quite a while. I've still got that can around here, use it for diesel fuel for parts washing now.