The process that made large washers used a special lubricating oil (aka cutting oil?) in the stamping so residual oil would be present. That may have been the only rust preventative on them. For restoration, I bead blast, lightly phosphate (1 1/2 to 2 minutes at 190F +) and then oil. It's is a better alternative than nothing, or wire brushing. I've seen a "bluish tint" on cut edges from the tool being over used. The oil was also supposed to remove excessive heat. At least in theory.