There’s another thing that can be done if galvanizing is not an option or the tank is already painted but needs to be restored. I was in the marine refinishing business for several years and worked with many of the leading industrial finishes that are not easily accessed by the public.
Awl Grip is a brand of paint that makes the super tough linear aliphatic polyester urethane line (yes I know quite a long title) but this paint is the toughest stuff around and probably close to what what used from the factory when they were new. Linear polyurethanes are the true polyurethanes. Products like Imron pale in comparison and are not true polyester urethanes! They are acrylic urethanes and are just basically glorified but fairly high quality acrylic enamels. You can tell a true polyester urethane by the mixing ratio which is 1:1 paint/catalyst. If you see a ratio of 3:1 like Imron then it’s not a true polyurethane.
Awl Grip also makes a very good epoxy primer which bonds to metals better that any other material including etching primers. The small tight long chain molecules of high quality epoxy resins are exceedingly strong, very adhesive in nature, and also water proof. Once a true epoxy coating is applied it will seal off all oxygen migration thus locking in any current corrosion making it deactivated and inert, plus closing off any additional H2O from penetrating from outside. This is space age technology. The polyester urethane topcoat will go over the epoxy primer providing an incredibly hard shell finish plus a gloss color coat. Both products can be hand applied by rolling and or brushing providing some remarkable results.
Do a search on “Awl Grip” and see for yourself.