Hi David
There is a product on the market that seals motorcyle petrol tanks, its like a rubberised soultion available from ebay
I have spoken to people who have used it before and they say its good, but very smelly as it is a 2 part sollution, mix it and swill it around the tank.
I intend to use this as I want to put a steel tank on my sweet pea
I hope this helps
Nigel