Storage for film photography can always be an issue if you’re an active photographer. And film negatives are susceptible to dust or mold if not properly stored. Negatives should always be stored in acid-free sleeves and archival binders or boxes in room temperature conditions to prevent potential damage. But things happen and you might come across negatives that need to be cleaned. Luckily, most common probems such as dirty, musty, or moldy negatives can be fixed.
Dirty negatives can generally be fixed with a simple can of air. Alternatively, a microfibre cloth can be used to more aggressively wipe a negative clean. As negatives can be rewashed repeatedly, washing the negative in room temperature water can also solve most issues. If you do rewash the negative then it is highly recommended that you use a Photo-Flo solution for B&W film or the appropriate stabilizer for color films. The negative should be hung to dry in a dust-free environment just like when it is developed and hung to dry.
If these basic clearning methods fail, you could try using a film cleaner such as Pec 12, which claims to be the only archival film cleaner on the market. If using a film cleaner it is recommended you try it on an unimportant negative to test it before applying it to one you hope to keep, just in case. Please note that any liquid added to the negative will make the emulsion more susceptible to scratching so always be careful.
When attempting to clean slide film it is recommended to remove the film from the cardboard carrier to effectively clean the entire surface of the film. Note that this will require you to have replacement carriers on hand, though this will allow you to transfer the film from an older, possibly dirty carrier to a fresh, new carrier.