I have been using molasses, for small things when cold a small crock pot. have a big tank but it is frozen solid. Going to try citric acid in spring. One thing you need when derusting is a phosphoric acid wash, your derusted part when rinsed will flash rust, a slosh for a minute or 10 will leave a metal prep finish. A good source of phosphoric acid is Etch and Prep by Kleen Strip, can be found at many paints big box stores. You can use phosphoric acid to derust but it does remove metal over time! I learned the hard way.