My car sat for 20 years and the fuel tank was all crudded up with biological growth, rust and would plug the prefilter fast.

I took the tank out which is easy to do, took apart & cleaned the sending unit following online tips.

For the tank I rinsed it out with acetone then went to a car wash & used the pressure wand to blast around inside through the sending unit hole. Poured in 2 gallons of muriatic acid, sealed all holes and rotated the tank to coat the inside. After 15 minutes of sloshing poured out the acid, rinsed well with water then poured in drain opener (potassium hydroxide – lye) to neutralize the acid. Rinsed well again then poured in Ospho Metal Treatment (Phosphoric acid) to give the interior a protective coating. Rinsed and drained again and finally poured in 2 gallons of denatured alcohol to absorb the left over water. Poured that out and the hot sun dried the tank fast,

The inside of the tank is now perfectly clean everywhere.
A days work gathering all the products and doing the job. Odds are even a used tank you get will need a good clean out.