Now this is on a much larger scale here I know, but the principle is still
the same. Some of the fuel tanks I inspect API 653, are only coated approx.
5 feet up the sides, to protect from water attack, the rest of the tank on a
floating roof style is typically not painted.
For the (3) 2.8 MMG million gallon Jet-A tank project I worked on for the
Honolulu Airport, we painted all surfaces with mil spec. 2 coat epoxy
system, as this was their cleanliness requirement. Moisture is the enemy
here, fuel will not corrode the steel.

Pat Thronson CWI NACE