As above regarding staining metal in the white. I have friends who have "wet" hands. The acids our bodies produce will cause the staining as well. I can attest to the fact that some of my friends can grasp a revolver, pistol and leave visible moisture of the grip frame. Their body fluids can cause rust overnight! You don't have to remove the hammer to remove the staining, depending how far down the hammer it goes. Take a flat piece of wood, (a flat ice cream stick works great) and wrap a. 400-500 grit metal polishing paper and work slowly and you will amaze yourself what a great outcome you will achieve, working slowly, keeping the paper flat so you will not"round" the edges of the hammer. Oil and your hammer will remain stain free for a long time.
Regards