I had a similar problem with an Australian tank that apparently rusted inside during shipping. My dealer boiled the tank and coated it. As a bonus, he powdercoated the outside. I haven't had a problem with it since.
LandCruiser FJ60 1986; 5.7 L Vortec, ARB Bull Bar, ARB Rear Lockers, Kaymar Rear Bumper, OME, 42 Gallon Gas Tank, Garvin/Wilderness Roofrack, BFG A/T, Optima Dual Battery System, White Knuckle Sliders, Superwinch Epi 9.0S, Yaesu 7900R, Adventure Trailer Drawers, Engel Fridge. On deck: Snorkel, WagonGear.
FJCruiser 2008; Garvin/Wilderness Roofrack, BFG A/T, Optima Dual Battery System, TRD S/C, ICON Stage I, TRD Quickshifter, some MPAC. On deck: Demello Bumpers, etc, etc.