Whats the problem? if its a leak then you can repair it with Plastic Padding Chemical Metal as this is chemical and fuel resistant, but you have to use it properly.

You need to cover the affected area and as its like filler its easy to apply, but preparation is crucial, I begin by stripping the metal with a wire brush on an angle grinder to remove all debris, followed by abrading with a coarse resin bonded disc on a backing pad on the grinder, and cover a large area around the affected area. Clean and degrease with cellulose thinners and allow to evaporate so the metal is spotless.

Mix a small quantity of the chemical metal and apply like filler but be careful as it goes off very quickly in warm weather and quickly is less than 2 minutes, longer in cold weather, once hardened you sand off any ridges and abrade the surface, mix another small batch and cover the previous area with another thin coat and extend this beyond the first coat, then repeat with a third coat. To accelerate hardening you can place a halogen light very close to the tank as these emit a lot of heat and accelerate the drying process.

With three coats applied you sand it smooth and paint the entire tank with a good industrial paint.