I had a small fuel tank, 6 gal ?, on one of my old tractors that sprung a small leak. A friend who restores Porsche's recommended I take it to the radiator shop and have them clean it and then tin the inside. Worked a treat. Apparently it's something they do to old Volkswagen and Porsche tanks to remove rust and seal. Wonder if anyone has tried that with cycle tanks.
All my cycle tanks that need lined for one reason or the other are done at a local radiator shop. They dip in a tank to clean, pressure test(fix any leaks) and then line. Cost of about $70 and is worth every penny.