The way i found to stop flash rusting is to empty the vinegar (neat vinegar - nothing mixed with it) then immediately fill with water from a hose and just leave the hose running to flush the gunge out. Stop the hose and pull off the tap and as the water comes out blast the inside of the tank with WD40 (as soon as the water has dropped an inch start the WD40 blasting).
I used some acid to get rid of the rust then used a Water hose as described. While the tank was nearly full of water but with acid flushed out I put some por-15 metal prep in. It coats the metal long enough to get the water out and you can get some gas into it. I think I threw some alcohol in at the end to get rid of any residual water in the gas. Tank came out dpik and span. I may have also sloshes around a little gas at first to mix and dilute the residual water while also coating the tank
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