I need some of the etched brass parts to assemble the rear axle assembly. I decided to blacken the whole sheet instead of the just the parts as needed for each assembly . This  is also the safest way to store them till needed. I searched the forum for blackening tips and after reading up,went with the following.  I started by carefully by sanding the front and rear surfaces with 1500 grit paper.My wife donated an old 8" x 8" baking pan. Using latex gloves,I filled this with enough acetone to just cover the sheet and cleaned off any photo resist and oil left on it. I left the sheet to dry,rinsed the pan and refilled with white vinegar. I left the sheet in the vinegar for 15 minutes,agitating and lightly wet sanding the surfaces again with the 1500 grit ,followed by a thorough rinsing. I then immersed the sheet in a solution of 7 parts water to 1 part BC Brass Black for 10 minutes. I removed the sheet,rinsed,wiped the film off and returned to the pan to repeat the process twice more until it was a uniform black after the film was rubbed off. It was then rinsed and returned to the pan with a baking soda solution,then rinsed again and left to dry on a paper towel. I am really happy with the results. I have used BC Brass before but I was never happy with the result,it was always crusty and not uniformly black. I now know it was not the product at fault,but bad prep and impatience on my part