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 was kind in sending one of the pencils, that he mentioned in his post, to me. I had the opportunity to try it this afternoon and I must say that it fills a need which I had experienced in using the two retouch dyes made by Retouch Methods. The dyes are very effective in adding density to large areas of the negative, but are limited in controlling density in small areas. This pencil is most effective in that usage. I particularly appreciated the softness of the graphite. I would highly recommend this product to all who are engaged in negative retouching.