[Floats]

1929,
It is a rubbery compound which is 'poured' into a clean, empty fuel tank. Then sloshed around, shaken, tumbled until the inside of the tank is covered with the stuff. Left to dry and the tank refitted.
Used as a bandaid repair for a leaking fuel tank. Works great for a while, if you are lucky. Works very well, if you are very lucky, but normally dont last for ever.