India ink was my go-to for a good while but these days I prefer a bit of renaissance wax with just a tad of lamp black for the color. Wipe it over the clean area and then buff lightly with a paper towel. Want it blacker, add another coat or two. Darkens the cuts and shading but not really noticeable on the metal flat surfaces. Easily removed and easily touched up and gives good protection to the metal.

Sam Welch