I just had a fun 6 days in the engine room and thought I'd share. Hobo has 2-350 gallon sheet metal tanks with no leaks (new 1987). I cleaned the port tank 2 years ago and was concerned when I saw the minor pitting on the bottom and the amount of sludge that had collected at the low end of the tank. This past week, after transferring all the fuel to the port tank, I opened the starboard tank and found about an equal amount of sludge and minor pitting. No surprise there.
The tanks have 3 baffles/4 chambers. All of the sludge/pitting was pretty much confined to the low area in the front chamber. I could see into the second chamber so I am guessing on the back 2. The tanks are 8 feet long.
After some internet research and looking at pictures of 2 tanks that had been coated, I went with FlameMaster 3204 A1/2. ($273/gallon)
Following the directions, I used a 4" brush to paint the bottom and sides up about 4-6". The stuff has the consistency, at room temperature, of honey with a 30 minute working time and a 30 hour cure time.
I didn't find any water in the last 5 gallons of diesel that I didn't transfer over. 15K plus gallons of fuel had gone through the tanks since the last cleaning. The engines never shut down due to clogged filters which were changed at 200 hour intervals.
Band-Aid or permanent fix? Who knows? But it's one less thing I'm worrying about.
The picture with the sludge, for perspective, you can see the blade of a 4" putty knife in the lower right corner.