Not knowing what chemicals and what percentages are in Ospho, I would try a 5/16-18 bolt that has been beadblasted in one ounce of solution to a gallon of 180-200F water for 3 minutes, a second bolt for 5 minutes and a third for 10 minutes. Air dry then spray with WD-40. Then add another ounce of solution and repeat with three more bolts. You will get different shades of "grey" which you can use to approximate the metal finish you want. Don't forget to use a stainless steel pan and tools.
Jim