Is your purple part sealed?
I didn't have any luck with that method. Some of the "splotch" on that part is the actual grain structure of the aluminum, which comes from casting or high temperatures from machining. Light sanding or sand blasting will get rid of it, but I did keep one part that had some interesting grains from machining.
To save your clips, use some aluminum wire or titanium rods to hang your parts and clip onto the hanger outside of the bath.