All steel objects are painted to keep them from rusting. The paint is a waterproof barrier between the steel surface and moisture in the air. Similarly, underground tanks need a waterproof coating to protect them from corrosive groundwater. Ordinary paint coatings do not hold up under constant contact with mineralized moisture in the earth. Specialized coatings are needed to withstand the corrosive environment underground and provide a barrier between the steel surface and air, water and even electricity without deteriorating over time.
The first line of defense against corrosion on an underground tank is a good coating. Paint and coating companies make a variety of coatings for a vast array of applications. It is important to apply those that will hold up over time to constant attack by water, oxygen, carbon dioxide, mild acids, mechanical abrasion from rocks, earth movements, roots, bacteria and other biological organisms as they feed on organic compounds in them. They must be applied correctly or they will not adhere to the steel surface. The tank manufacturer often applies a tough, chemical and water resistant coating at the factory. Factory applied coatings include baked on powder paint, sprayed on chemically cured epoxy resins, electrostatically applied fusion bond epoxy and sprayed on coal tar or petroleum based enamel. Field applied coatings are usually brush applied enamels, mastics, polyurethane, reinforced plastics or epoxies. If you have an underground tank or are planning to install one and can scratch off the coating with a pocket knife, it will not prevent corrosion underground. You should apply a second coating made for underwater or underground use. If you are located far from a large metropolitan area and do not have the luxury of applying one of the above, you may find one made to resist water in stock at a local boating supply store.
Coatings alone, will not protect a buried tank against corrosion. They are scratched, scraped and gouged during handling, setting, backfilling and settling. They can have small imperfections or missed spots here and there. Some deteriorate underground. Corrosion pitting rates are greater at these flaws, scratches and gaps in the coating than they are on completely bare tanks. Cathodic protection with anodes is needed to protect the steel wherever it is exposed.