I guess, it all depends on the water/acid dilution.
Last weekend I went to one of the family oriented home-centers and bought a gallon of concrete cleaner/etcher (hydrochloric and phosphoric acid) to clean the inside of my radiator.
After doing some experiments with various metals, I decided to use full strength, because the dilution seemed mild to medium.
I ended up leaving it to soak over night.
Removed about a cup full of scale and rust build up and the metal was practically like new.
Rinsed with washing soda and was ready to go.
I bought this radiator a little while ago, after it was sitting in a garage since the 50s.
I cleaned the entire engine cooling system using radiator cleaners from the auto parts store.
After a few trips on the freeway, on a very hot day, I noticed that it didn't cool down as well as my previous leaking one.
But after the acid soak the radiator seems as efficient as new.
Just goes to show how important it is to keep the coolant passages clean so the fluid can absorb the temperature of the metal more efficiently.
Since this cleaner seems safe for the softer radiator alloys, it should be safe on fuel tanks even in full strength.
But only careful experimentation can tell for sure.
