What is a proper way to clean a 35mm negative film stripe before scanning?
What chemicals can remove fingerprints, dust that does not come off by blowing, or strains of glue? What type of cloth does minimal damage when rubbing off dirt?
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For fingerprints and light dust you can use a microfibre cloth. If you have stubborn dirt, dust (or glue!) then you may scratch the emulsion if you rub with a cloth.
In that case you could try film emulsion cleaners, but I would just soak them in lukewarm water, perhaps with a single drop of dishwashing detergent to prevent streaking, then use a soft squeegee and hang to dry.
If there are photography stores in your area that still sell darkroom supplies, there are chemicals and tools specifically designed for cleaning film. You can see a sample of what I mean at B&H Photo which can help you find sources in Austria. If you have lots of film to process, you might be willing to pay for a machine dedicated to it.
Never underestimate the cleaning characteristics of clear water. Handwarm for your convenience.
User your fingers (within the water) for rubbing things off. By doning so you can best control whether it works fine or damages the emulsion.
Some drops of wetting agent are quite unexpensive, better than dishwashing detergent although both share the main ingedient.
Using demineralized water (calcium free) reduces the risk of water (calcium) spots while trying. Then you may not need any wetting agent at all.
Ugh... NEVER clean film negatives with water. Use dry microfiber cleaning cloths, or - if you must use a liquid - try the PEC-pads, which are disposable microfiber wipes with the PEC-12 solution already on them. B&H has 100 packs of the PEC-12 4''x 4'' (10cm x 10cm) wipes for ~$8, as do many other photo-supply shops (and amazon, et al).