What to use for parkerizing touch up?

[tdbru]

Croak is correct. Aluminum forms a surface oxide upon exposure to atmospheric oxygen. you could just leave it and it won't corrode or rust like iron alloys. and the Lower is probably Anodized, not parkerized, anyway. usually parkerizing is for steel alloys, anodizing for aluminum alloys. the only reason you might want to cover it is for appearance. and as pointed out, sharpies, or paint, or Birchwood Casey's Aluminum Black, they all work.
-tdbru