Rust is iron oxide and all oxides are basic so an acid will react and dissolve the oxide, but strong acids will also dissolve the metal. In particular, common muriatic acid (30% HCl) will really dissolve iron easily! That is why vinegar works OK up to a point, it is only about 3% acetic acid and acetic acid is a weak acid and finally the results are acetate salts soluble in water so the vinegar produces iron acetate which can be flushed away with water. This is a good trick for large iron objects where you don't miss a little of the iron which is dissolved but could cause damage in a copper or especially aluminum radiator. That is why the folks who flush radiators use a basic solution rather than a weak acid. However the basic solution used to flush radiators will not have much effect on rust. After that long discussion, the case at hand for me was to flush out a (MG Midget) gas tank with rust in it. A thorough cooking in radiator flush process did little to remove the rust which presisted so I should have tried vinegar at the risk of creating pinholes in the tank. Instead I put scoops of gravel stones in the tank along with a short length of 1" chain and shook it up followed by rinsing but still I had rust that was so bad that I had to carry spare fuel line filters for on the road replacement. A better solution would have been to use vinegar followed by a tank sealer but I tell this to remind the vinegar users that it is a tradeoff of cleaning out rust versus creating pinholes. Conclusion: vinegar will dissolve rust but radiator cleaner basic solutions will not, but on the other hand thin metals in radiators could be damaged by vinegar so that is why basic solutions are used on radiators. Some metals like aluminum form oxides which will dissolve in either an acid or a base (amphoteric oxides) so the gentler basic cookout will clean radiators but have almost no effect on rust. Second conclusion: don't use vinegar on radiators.

Don Shillady
Retired Scientist/teen rodder