Rust Converter Recommendations. Aerosol for sun faded/surface rust car.

Ddaddy said

Phosphoric acid converts rust (orange oxide) to black oxide that creates a barrier to further corrosion. That’s what the POR products use to achieve their results.

Just spray with phosphoric acid in the nooks and crannies and carry on.

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Yep, that’s what I use, cheapest best option is to just buy a large container of Phosphoric Acid!
The (originally very rusty) exhaust manifold below was cleaned up with nothing more than phosphoric acid, rinsed and finished in satin clear heatproof.