Kodak makes a great product called Kodak Retouching Fluid (Cat. No. 195
6309). A 1/2 oz. bottle is $4.00 or so and will last a **long"" time.
Modern sheet films have very little "tooth" on either side; consequently, a
pencil mark won't adhere to the film. The Kodak fluid is a very thin clear
varnish which dries with a flat finish that has a microscopically rough
surface and will hold a pencil mark. You put a small drop on the area to
retouched (it works on either side of the film) then spread it very thin and
feather the edges with a cotton swab or your finger tip. If you want to
build up density in an area, you can put at least a few successive coats on
an area to allow several thicknesses of pencil marking.
If you are enlarging negatives, don't forget the opportunity to retouch on
the interpositive.
A few years ago, I found in a used book shop two old books on retouching B&W
negatives, dealing mostly with removing "blemishes, lines, and shadows" from
portraits of "mature" subjects. ("The middle-aged subject with still
youthful features will appreciate a little help from the retoucher.") The
approach is a little dated, but there is a lot of useful information in them.
There must have been many other similar books in print at the time. These
books are:
Draper EE & Jarkness N: Negative Retouching and Print Finishing. Little
Technical Library, Ziff-Davis, 1941.
West K: Guide to Retouching Negatives and Prints. The Modern Camera Guide
Series. Greenberg Publisher, 1955.