One of my coworkers recommended this video to me, and I respect the video host's adherence to the scientific method. He tests metal from the same source, prepared in the same way, and has multiple test samples for each coating.
I'm not so sure, however, what these rust convertors actually do. I found this in the wikipedia article on rust converters...
Commercial rust converters are water-based and contain two primary active ingredients: tannic acid and an organic polymer. Tannic acid chemically converts the reddish iron oxides into bluish-black ferric tannate, a more stable material. The second active ingredient is an organic solvent such as 2-butoxyethanol (ethylene glycol monobutyl ether, trade name butyl cellosolve) that acts as a wetting agent and provides a protective primer layer in conjunction with an organic polymer emulsion.
Some rust converters may contain additional acids to speed up the chemical reaction by lowering the pH of the solution. A common example is phosphoric acid, which additionally converts some iron oxide into an inert layer of ferric phosphate. Most of the rust converters contain special additives. They support the rust transformation and improve the wetting of the surface.
Looks like they're primarily tannic acid with some organic solvents.
The science seems pretty interesting, and I might show this video to my students when we discuss experimental design methods.
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