Tank coatings

Diesel and Oil tanks can be steam cleaned, they should be treated after drying by wiping over with the fluid they will carry once full.

I have 1942 steel tanks. If you keep a good conditioner in the fuel and circulate the fuel often, you can keep the water and bugs out of the tanks. When I opened mine, I had some deep pits in the bottom. After cleaning I welded the pits. There are epoxy paints for tank coating, but they require clean, sand blasted metal.
My tanks were built w/o entry ports so hadn't been entered since built. I used them for a couple years before I got to that project. I always use diesel additives are biocides and help separate the water from fuel so your primary filter can catch it. When I opened the tanks they were clean, no sludge, no noticeable water. Pumps left about 2" of fuel that had to be removed with a portable pump.
If some of the tanks aren't used for long periods, they should be completely empty and the metal treated. I've seen tanks in mothballed ships that disappeared with sand blasting.

In my experience the answer to that is a big yes, I would not paint fuel or oil tanks myself and can't remember being on a ship where they were painted.

Most of the fuel tanks on ships I have been on have been painted internally. A good reference for the prep and paint specs can be found in US Navy NavSea publications. Likely the same paint specs used by the CG.

Like the internals of larger engines, painting provides a smooth surface so that contaminants are less likely to adhere to the tank sides and bottom as well as provides corrosion protection. Many of the large slow speed engines are painted white internally to make inspection easier and make it a little less like a coal mine when working inside them, fuel tanks are usually painted in that red/brown color that already looks like rust.

That may be changing Capt J, I've replaced a couple sets of aluminum diesel tanks in the last two years on boats that were less than ten years old. These tanks were all coated on the outside and properly installed but the corrosion and pitting that led to failure was on the inside bottom surfaces. Apparently what happened is microbes grew in the water collected in the very bottom of the tanks and their excrement is acidic and eats into the aluminum eventually resulting in failure. Our local tank cleaning service tells me he's been seeing an increasing amount of these problems. An internal coating should help if not eliminate this problem on all types of fuel tanks.

I have seen tanks in long mothballed naval ships being returned to service that have rust so bad that when sandblasting the tank wall you suddenly are looking into the next compartment with each sweep of the gun. In most cases the navy closed that one up and picked another ship.