Has anyone used 3M's Scotch-Clad 776 to seal a plastic tank? Its used in the aircraft industry. Avail on ebay,tempted to give it a try?
Used JB Weld Marine on a hole that happened while fixing spinning tank nuts.
It's been a few months and when I say hole I mean hole.
I let it cure for 2-3 days before adding fuel.
I assume you are just "lining" the existing tank, not repairing a leak?
These guys have a respected line of products.
http://www.caswelleurope.co.uk/gastank.htm
I've not used them, but they are a respected name in the classic car arena