You can get a kit to strip out the current lining of the tank and put in a new one
its a three part process of putting various chemicals in the tank and sloshing it around for 1/2 hour then washing out the final process is the new lining
ive done it on a couple of tanks now and seems to hold up well
here is a link to the kit ive used but as im in the uk may be something else available closer to you