"Rust converters" are a legitimate treatment - within their legitimate scope of use. The industrial products I have experience with pretty much perform as advertised. The basic chemistry uses tannic or phosphoric acid to react with the iron oxide (rust) to produce stable ferric tannate or ferric phosphate (respectively) as the reaction product. These are pretty much stable and are suitable for coating.
I only suggest its use where conventional abrasive blast or mechanical cleaning cannot be practically employed due to site conditions and/or cost.
The trick is, the acid solution must come in contact with the rust. The products I'm familiar with also contain an organic solvent to promote the penetration of the solution. In practice, this means mechanically removing as much rust as possible prior to application. It accomplishes nothing to treat the outer layer of flaking rust. This also means that oil, grease, paint, etc. must be removed prior to application.
An ideal application scenario is where the rust on the surface, with no or little flaking. It's not going to help a lot with cancerous and deeply pitting rust. There's a NAVSEA (Naval Sea Systems Command) study out there from 15-20 years ago that compares half a dozen or so of these products in maritime lab simulations, almost all of which are still on the market. Conclusion - some are better than others, depending on acid strength and the effectiveness of the organic solvent.
This was discussed within the past year or so in a TF Maintenance thread, but durned if I can find it. I want to say that FF posted a "home brew" of liquid dish detergent (organic solvent) and muriatic acid that does the job.
