Was it not phosphoric acid, which is the active ingredient in most rust converters? I use hydrochloric (pool acid) to remove surface rust, but unless neutralised with an alkaline or plenty fresh water, it starts rusting almost immediately. Just my 2c, I could be wrong.
When restoring old petrol bowsers, I would completely immerse all the cast iron bits in a strong solution of caustic soda. Not a trace of paint or primer left behind. Just make sure there are no non ferrous metals still attached, as it will quickly chow that up. Again, rinse well before painting, and paint asap, as it starts to surface rust very quickly.