That depends what's wrong with it. My standard solution for rust flakes and general gloop is to remove and blank off the fuel gauge, put a shovel full of gravel in, strap it to the drum of a concrete mixer and give it a good churn. Turn the tank round and do it again so you don't miss any. There's a video somewhere on YouTube. Obviously getting it all out again can be a problem. Once dry you could line the tank with "the relining type stuff", but I have no experience of that. I welded up the rust holes that showed up, but I can't recommend tha,t as I had access to specialist extraction equipment to avoid being blown up.What would you recommend to do with the fuel tank?