Restoring old (Hooker?) race headers
These are nicely built headers with 1 5/8" tubes and 3" collectors. These were probably uncoated steel since they developed good amount of surface rust after years of being in the open. Surprisingly they were still solid!
And now, here's the science: You can see 3 distinct(ish) states. The flanges and first 1/3 of the tubes are bare steel. The rust has been turned into black iron phosphate by phosphoric acid immersion, which washes off easily and falls to the bottom of the acid/water bath. This is what it would look like sand blasted, but less metal has been removed by this process (I think). The next area is partially converted rust into black iron phosphate. The last area by the bottom (the header collectors) has been converted, washed, dried and coated with high temp (1000f+) paint.
This header has been in a 15% solution of phosphoric acid and water for about a week. It was probably completely converted in 2-3 days, but I couldn't find any time (!!) to pull them out for washing/drying and painting. You CAN leave the iron(III) phosphate (FePO4) on without washing, but it is a porous crystal structure, so it will absorb water and transport O2, hydrogen to the iron layer, which will rust eventually. It is a good base for paint as long as its neutralized and completely dry. I believe that would entail ammonia and then solvents for drying, but it's easier to just wash down the steel with a brush and dry it off with a torch.