Well after just 15 hours The mill scale is completely removed!
This was just basic white vinegar and it did an awesome job. A quick bush off and scotch bright brought out the true steel surface.
Within 5 minutes of rinsing in water the plates began to flash rust! So be sure and get some oil on them right away. Here is a shot of the vinegar after it has done its thing.
Here is a comparison of the two looks. natural mill scale on the left and scale removed with vinegar on the right. I prefer the natural mill scale look on the left.
Removing the mill scale does open up other possibility, such as bluing, or perhaps engine turning.
Video of the test results here