I had to replace the fuel tank on my '97 Lumina 3.1 150k miles as it
>developed rust holes 1/4 inch above the pinch weld in the back where the
>weld is bent up some to allow for the straps. Apparently this area holds
>road grime (and moisture) quite well.
>
> Symptoms were a fuel smell, yellow stain on the tank exterior, plugged
> fuel filter, fried fuel pump, high long term fuel trims under load
> (+19.5%), and a bad surging misfire when first restarted hot that made my
> aftermarket tach swing between 500 and 1500 RPM about 3 times per second.
> Revving the engine once usually cleared this up but then it got worse to
> the point that when put in gear the RPM fluctuated so rapidly it made the
> serpentine belt chirp and a couple times even caused a flashing MIL and a
> P0300 random misfire code. Oddly no EVAP leak codes were present. This is
> per an OTC 4000 Enhanced scan tool and the Pathfinder '96 software. >
> Post mortem teardown inspection of the pump revealed that it was made in
> '03, the commutator was fine, but the copper flex wire from the positive
> terminal to the carbon brush was completely gone so the current had to go
> through the spring instead which acted as a resistor and slowed the pump
> down and the heat both made the spring lose its tension and discolored the
> plastic top of the pump around the terminals. I can't believe the pump
> assembly costs over $300 new.
>
> For now I have in a salvage yard tank and I cleaned off the surface rust
> in the areas that rusted through on the other tank and smeared on some
> good grease to hopefully protect the metal. I plan to install a new tank
> in the spring.
I learned years ago that moisture and on metal causes rust.
So you need to clean it ALL (rust) off with a wire brush or such, scuff rest of it with sand paper, and put a two-part zinc primer on it, then paint it with automotive enamel. And try to keep it free of mud and stuff afterward.
Trying to isolate it from ground won't help, if you could suspend a metal plate in the air in the eastern or southern parts of the US, it would still rust.
Anybody living in these areas of the country and needing a used fuel tank might do well to get one from a wrecking yard in Vegas or Phoenix, not much rust on cars here, and I never heard of anybody replacing a fuel tank due to rust.