Reply to "Gas tank"

OK. I derusted an old gas tank using a 10% solution of Phosphoric acid, which is a so-called 'weak acid' sometimes used to etch concrete. Weak acids tend to attack iron oxide (rust) but not steel. Strong acids such as hydrochloric or sulfuric tend to do the opposite. Just dumped it in & filled the tank to the top, taped it off and let it sit for a day. Upon emptying the stuff out (hazardous waste, remember), the rust was gone leaving a silvery-white coating of iron phosphate. I dumped some light oil in and sloshed it around to cover the stripped tank surface & prevent re-rusting. 6 months later, I installed the tank- no further problems.