Fuel tanks external surface rust treatment

[Hardship]

When I bought my 2003 boat some years ago, the survey noted some surface rust on the bottom and top of the mild steel tanks which should be treated. The tanks are outboard of the twin engines. The bottoms are accessible to a point as they are supported on bearers. Some faces are not accessible. There have been no leaks and all decks are sound and the engine room is now fairly pristine. For winter, I have removed various items for good access to work on my engine mounts, so now is the time to do something. I am reluctant to use paint as this will inevitably make a mess of surrounding surfaces. I am thinking of applying rust convertor, then a clear waxoil / Lanoguard vehicle underbody type treatment, applied by small roller to all accessible faces. Has anybody done this? If so, I would appreciate any comments.

 

[Mac2]

Ospho is my favorite converter. You don't have to rinse it after application. You just need to wait for 24 hours before painting. In areas you can't reach, you might want to consider one of the spray containers that people use for killing weeds (plastic tank with a hose attached to a wand). It's the type you hand pump to pressurize. You can adjust the spray nozzle for the areas you want to spray and the wand allows you to reach areas you can't get to by hand. Even if you can't get to the areas you sprayed with Ospho, it leaves a nice hard coating that should protect the tank for awhile.

 

[Comodave]

I have had good success with POR15.

 

[Andy G]

We had a very similar story. Twin engines with outboard saddle tanks, with some rust noted.

We used a rust buster followed by metal primer. This appeared to work for a couple of years but was really just a holding operation.

Ten years later we have just finished replacing the tanks. What was interesting was once the tanks were out we could see the major areas where the tanks failed was where they were sitting on the bearers on the outer side of the tanks.

A recomendation is to buy a cheap endoscope, it will let you see areas that otherwise you could not visually access. You can get them for about $80-$90 on line.

It was amazing how well our old FL 120's could tolerate the rubbish that built up in the tanks, I doubt that some of the more modern engines could have put up with this treatment.

Good luck with the tanks as repacement is expensive

 

[Bmarler]

My go-to for surface rust is picklex 20 rust converter and zero rust paint. You really should find a way to temporarily remove the supports if at all possible, or lift the tanks a bit. Big ask, I know.
With picklex, you just wire brush the surface to remove loose rust, then use a scotchbrite pad and scrub it thoroughly, wipe off excess.
Zero rust paint is used by bridge maintenance crews to coat steel bridge frames. I use it for car restoration chassis work. Comes in different colors and is fantastic for coating treated or untreated steel. Roll, brush, or spray.
If paint just isn’t in the picture, I like fluid film. It flows well and self heals. I think it’s lanolin based, but I’m not not positive. I use it on bare steel that won’t be painted. Protects and lubricates.