Try handling the negatives as little as possible, especially pushing or pulling them through plastic sleeves; fine base scratches may result. And NEVER pull a negative strip through a film gate, no matter who says you can; you will scratch the film like you wouldn't believe. Never clean negatives by pulling them through your index and middle fingers; I pull them, with very light pressure, through a folded Ilford anti-static cloth. Insert the negative into your negative carrier, dust it with a jet of air, and load it into the enlarger.(posted 8839 days ago)Your camera may be scratching the film, and the best way to check for that is to do a "scratch test." This test is often used in any environment where film is transported, such as motion picture magazines, film splicers, et cetera. To make such a test, run part of a roll of film through your camera, and without disturbing the the film, open the camera back. Inspect the film, and if you find a scratch, the film will still be "aligned" so that you can track down the source. It MUST be a fresh, factory-loaded roll of film. Do NOT USE bulk film (see below).
A common cause of scratches is the film pressure plate, which can cause base scratches. An emulsion scratch, which is a scarring or an outright gouging of the emulsion is sometimes caused by dirt or grit caught in the camera. I once searched for a deep emulsion scratch for hours, until I realized that one of the two flat springs which held the pressure plate on one of my motorized Nikon F cameras had sprung out of place!
Dirt on the felt lips of a film cassette will cause either base or emulsion scratches. If you get the lips dirty, the best solution is to reload the film into another cassette, and toss the dirty one into the recycling bin!
A word about bulk film: I had fine base scratches off and on until I stopped using bulk-loaded film. The film was clean; my cassettes, some of which were used during my student days in the 70's (I'm dating myself) were the culprits. As soon as I started using factory loads, my problems with fine base scratches disappeared.