This topic comes up often. What I've found for the cheapest cost. For dunking small parts citric acid. For large parts you can't soak and pitted rust after a stripping disk I use. rustoleum gel. 32 oz, can buy it at Menards. $10.57 or Amazon $11.97. I did my truck cab and 2 fenders.
I've found that Muriatic Acid followed by a thorough rinse then sand & primer works better than anything else I've tried. I don't like using it inside my garage as the acid vapor will make any other raw steel in the area rust quickly. I learned that the hard way and now do any acid rust removal way out in the back yard.
Walmart sells a stronger grade of vinegar then the local grocery grade, can usually be found in the pool supply area but much more costly I might add.
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