I just did this successfully using some solutions and paint from Frost auto restoration. They are based on the POR-15 tank sealer system. The first
(marine clean) strips out rust lacquer and gummy deposits, the second etches the cleaned surface ready for painting, the paint is a specialist paint
for fuel tanks. These treatments cost about £12 each (<£40 total) and you need to take the tank out to do it. I would
recommend that marine clean can be omitted if you first give the tank some caustic washes: I used caustic soda (cheaper and probably more effective)
made up to 1 litre for paint stripping (instructions usually come with the caustic) and did three washes with water rinses and a citric acid (lemon
juice) or vinegar rinse after each. Leave the caustic in for about 4 hours each time swirling it around to cover all surfaces every 30 mins or so. This
should clean out everything undesirable ready for etching and sealing. Good luck
Josh, Colin,
Steam cleaning is the daddy. You can even weld a tank after that. Look in Yellow Pages for steam cleaners, or ask at any firm that runs a fleet of
lorries. If there is significant rust inside or out, also get the tank pressure tested.
John