Yep, ethanol isn't kind to bikes, this is a tank off a Fugly BMW R1200C (BMW's version of a Cruiser from back in the 90's) that the female owner ran with 5% ethanol fuel, then left the shidt in the tank over winter.
She took it to a motorcycle shop (not a BMW dealer) who told her that the tank had pinholes, hence the leak, and offered to replace the tank and pump for only $4500, plus labour.
Once I'd cleaned it out, (POR-15's phosphoric acid initially to get rid of the big rust deposits) I filled the tank with white vinegar and let it soak for a week to clean off the lighter rusty residue, and surprisingly (not) the tank didn't leak, so, so much for the dealer's "pinholes".
What had really happened is that the ethanol swelled the "O" ring that sealed the fuel pump assembly's baseplate into the bottom of the tank, causing fuel to bypass it.
The only problem I have now is that the OEM BMW liner is falling off in big chunks, and I'm trying to work out how to remove it without any chance of damaging the paint on the outside of the tank. Any ideas? Cheers, Terry.