Hello,
I have a box of pencils in my desk here at work that I use to write on
drafting mylar and clear film. Actually, printed on the side of the pencil is "writes on: glass, plastic, film, lacquer, metal, paper,wood, etc.
You can get them through any office supply store that special orders.
And they are: "KOH-I-NOR" KOH-IN-ALL No. 1555
They're very soft and with the aid of some fine grit sandpaper, here in the drafting trade we call it a sanding block, you can get a very fine point. They remind me of my wifes eye brow pencil.
I've got a dozen here in my desk. If you're serious about retouching and would like to try one of these pencils, I'd be happy to send you one. Send me a separate email. You have to promise to give a report back.

Rick