Clean tanks seem to be toughest to find and they're often in bad shape. On both my pre-gens, tanks were rusty with one being extremely thick and foamy rust. Following some guides from vintage bicycle forums, I soaked them in oxalic acid (easy to find and use). Little did I know that oxalic acid eats both rust and good metal at same voracious rate. I ended up with holes in both tanks.
I bought two more tanks, both equally rusty. This time I researched topic a little more and used phosphoric acid on one of them (available as concrete-cleaner/rust-remover at hardware stores). Phosphoric acid preferentially eats only rust and stops at clean steel, leaving mild protective coating. I put tank on spit and rotated 90-degrees every couple hours. I went through 4 pint-bottles of it on this tank in 2-weeks and was left with nice clean metal! Coated it with POR-15 then followed up with diluted epoxy resin.
On 2nd tank, I tried electrolysis with washing soda. That worked pretty well and was faster than phosphoric acid. Also POR-15 coated and sealed with epoxy resin.
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