Actually, the best thing to use is a retouching pencil. Ohhh, my gawd I hate retouching negatives. I had a retouching class, 2 hours once a week for a year.

Bleeechhh... you really have to have a "touch" for it. I never did get very good at it. Prints? Oh, yeah can do that one easily - color or black and white. Film?? Did someone say - Film??

The best way to do it is to get retouching fluid and retouch on the back side using the retouching pencil. The fluid will give the slippery surface some "tooth" to grab the graphite from the retouching pencil. The retouching pencils are some special formulation (I have no idea what they add to the graphite) that is gooey enough to stick to the film surface.

From my experience, (poor as it is) - you need to "stipple" the area in question with little dots and NOT try to fully fill in the scratch. There is an upside / downside to working on the backing side (hmmm...almost a limerick in there somewhere).

The upside is it the retouching is easily removed if needed, and acts somewhat like an unsharp mask in softening the transition between your retouched area and the surrounding image.

The down side is you have to not quite fill to the very edge of the area to prevent a halo being formed around the retouched area because of diffusion through the film base. Oh, yeah that's easy to calculate...that's the "touch" part.

That statement is predicated on the idea that you're skilled enough to have matched the density of the area closely in the first place (haaaaa....haaaaa...haaaa - in my case).

If it's a really important shot - pay a professional film retoucher. That's what I do.