If you are in the UK it would be worth contacting Northern Radiators Ltd they offer a process called Fuel Tank RE-NU , the tank is thoroughly cleaned before a plastic lining is applied inside and out .
I have not used them myself but a friend of mine has on two tanks about 4 years ago and has been totally satisfied with the results, if the tank is very weak in a few area a specialist tank repair company could let in new sections and then Northern Radiator treat the tank for a belt and braces job.
Radiator Repair Northern Radiators Leeds NRG
Geoff.