Super glue and baking soda. No Joke.
I have a Stihl chain saw that had a crack in fuel tank. I tried all kinds of things and nothing worked permanently with the high revving vibrations and fuel proof. 2 years ago, I did the Super Glue and baking soda on the crack...still leak free.
https://www.youtube.com/watch?v=QFrfRxnpdBQ