Obviously an alloy welder would be best, after copious flushing with lots and lost of detergent and water
Other wise If it's a temp fix a Good Clean down to bare metal and JB weld or Devcon A for ally or Posiflex Sealant (Superb stuff that) and You may get more than a Hundred years or ya may not???
Drain the tank, ventilate well (safely) till totally dry. Pour Clear Bostik Glue into the local area. Support the tank at the correct angle to make sure the crack is wetted. Won't be ready for tomorrow though as you need to let it set, but give it a few days...
Cheers lads for advice
Its the lads gasgas trials bike that i damaged went out looking for jb weld but none in or local , ended up with something called "petrol patch" not sure if its worked yet will have to get up early in morning to try it and assemble bike for his trial.
Thanks for all the posts and help. Used petrol patch as only thing in the shop at time and it was getting late , seems to have worked and he enjoyed his ride on sunday
All your ideas are handy to know and have now thrown a tube of araldite in toolbox and chew bubble gum