I have also had this problem with a number of different cameras and intermittent across the negative frames. To try and fix this I cleaned the cameras thoroughly before loading film. I develop in Caffenol C-L and do not squeege, but shake the water off before hanging from hooks horizontally on a modified bike wheel which hangs from my camera room ceiling. I just could not see how I could be causing the scratches. I then read that dust can lodge in the film canister felt and as you rewind this can scratch a straight line in the film.
However my last roll of film had a dead straight line from frame 3 to the very end pretty much ruining all the shots. The line was on the outside of the film, not on the emulsion side. A thought came to mind to crack open the canister and check the tag of film remaining there. ( I cut off the end bit of film after threading it onto a Patterson reel in the bag). Now this tag has not left the canister so it's not been in the camera, or been developed, or been exposed till now. And there it was, the scratch had gone all the way in and I could see it without a magnifier lens. So it was on the film before it went into my camera. I then cracked open about 6 more canisters and found faint lines on 2 of them.
It's probably fairly rare that this is the problem and more likely to be the intermittent dust scratch lines, but if you've discounted all possibilities then check the film left in the canister.
Just to throw another curve ball into this thread, I have another theory on those lines that are on the outside of the film. I load my Patterson reel by pulling down on the canister, winding up till the canister reaches the reel and repeat the process. Now this means the film in the canister is not under load and hence springs outwards so putting the outside of the film up against the inside of the canister. Do you think the unsprung film has potential to be scratched by any imperfections inside the canister wall using this method? Do you think a better method is after starting the film on the Patterson reel, to then crack open the canister and remove the roll of film first, before then winding onto the reel, so eliminating any chance of scratch lines from dodgy canister internal faces?
The scratch line on this negative is 20mm down from the top.
