The black is caused by the carbon from the steel when the bonded iron oxide is removed from the steel. The evaporust takes the iron but leaves the carbon. Here's a great break down from a chemist with Red Kote products. I was asking if Evaporust would work with the coating.
"I was very apprehensive about this product when I first heard about it, because I believed it to contain phosphoric acid. We do not recommend phosphoric acid, as it is a “rust converter”. It would need to be scrubbed off, before Red-Kote can be applied.
Evapo-Rust, however uses iron chelation technology. The chemicals bind the metal ion so that the metal ion cannot interact with other elements. When the ion is bound, this creates a complex molecule that can be washed away from the surface."
Worked amazing on my motorcycle tank.
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