I well remember doing this when I was a kid....I left it to soak overnight, emptied it out...and took it to the fella who was going to weld it...
"here"... he said..."here's the torch" (lit) , ...."before I start...I wanna see you stick this inside the tank through the filler"
me... being naive as ****, and with the bravado and stupidity that only a teenager could have, took the torch...and shoved it into the tank....
WHOOOOOOOSH!
a blue flame shot out of the end, the tank span around on the garage floor.....and I nearly shat myself....other than singed eyebrows there was no damage done...
meanwhile...matey (the welder) couldn't do the job for 20 minutes....he was too busy pissing himself laughing....