An old, painted steel tank is practically worthless except to paintballers. However, if you insist on using this tank, the interior must be cleaned of all rust/particles and oil residue. No coatings of any kind. The interior can be cleaned with Dawn detergent with a pound of marbles added. Roll tank across the yard, drain, rinse, drain and blow dry with hair dryer with long extension or similar. This is the budget, tedious, flash rust solution. Otherwise, purchase some Ensolv oxygen approved solvent and rinse out the tank, drain. No rust, no marbles, no blow dry. This stuff may cost more than your tank is worth, however.
Sand blast or strip the exterior. Coat the exterior with Zinc Rich paint- CRC, Rustoleum or similar type. Compared to hot dip this is a relatively weak, cold galvanizing but actually works although must be done yearly for SW use, every two years for FW.
