The Oxpho Blue works fine; you only need to follow the directions - without trying shortcuts. It penetrates deeper than a permanent marker (which of course doesn't penetrate signicicantly). I've not used a 'touch-up' pen.A black permanent marker sometimes covers small spots.
VT How well does the Oxpho Blue or the touch up pen work? Does acetone remove any of the other bluing? Would rubbing alcohol be safer or worse to use than acetone? You are just trying to remove any oil or residues correct?