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Join Date: Jan 2011
Location: Smack in the middle of Illinois
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The white stuff is corrosion (oxidation) caused by moisture (most likely condensation)
The only way I know to keep rust out of a fuel tank is to keep it full of a non-corrosive liquid or to place it in a sealed container void of oxygen (plastic bag filled with nitrogen)
Fuel (gasoline) is not good enough - it contains oxygen - diesel fuel is better as it contains less oxygen
If i was trying to preserve the tank, I would clean it with acid to remove all traces of corrosion, neutralize the acid, rinse with water to remove all traces of the acid, rinse with denatured alcohol to remove the water, dry it in 120 to 140 degree oven for 24 to 48 hours then completely coat the inside of the tank with a non-oxygenated oil (fogging oil for example), then place the tank and a couple of bags of desiccant moisture absorbent in a sealed (zip lock) plastic bag.