I came across a collection of family B&W negatives (35mm) that have been stored rolled up individually inside their canisters for 35-40 years. They are all
severely curled. I did not realize initially how drastic the problem was and proceeded to cut many of them into strips and put them into sleeves, thinking this would force them straight after a while. I was so wrong. Putting them into sleeves did nothing. Some of them were so curled that they would not only curl the sleeves they sat inside of, but also tear the paper of the sleeve. I them realized different measures need to be taken and placed all the sleeved films flat between large hard cover books with a large weight on top. I kept them like this for months, 3 or 4. This had NO effect on the curl.
I then took to the internet to find a solution. I was very confident there is at least one that is guaranteed to work and I was blaming myself for not doing this in the first place. Turns out there is no tried and true method that works consistently to straighten badly curled film. Some people tried heat and claimed it works, others had luck with simply keeping the films flat for a while. I did not want to cook my film in the oven, like others did, and risk to ruin it. What I did was wound the film strips onto a thin glass jar, inside which I put hot water. Not boiling hot but hotter than it would be comfortable to touch. I don't remember the temperature but I know I used a thermometer and was consistent about it. This method had some success but did not cure the curl completely. And I gave up on it because I was worried that the heat will risk to eventually compromise the emulsion, especially since the film was so old.
If you've read this far, know that in the end I tried a solution that worked for me. And it worked unexpectedly well, especially for how simple it is compared to everything else I tried. The trick that worked was rolling the films backwards with a similar tension (aiming for the same diameter of the roll) and keeping them like this for a few months. The films that were still uncut straightened up beautifully with this method. Sometimes I noticed a very moderate reverse curl, which tells me you can overdo this. I have no way of knowing how much time is enough though, so I took a chance.
TLDR: If you haven't yet cut your film into strips, roll it backwards and wait a while. If you did...well, do the same but it's going to be a lot more difficult to do it
Edit: added the film format, which I forgot to mention initially.