Dear gobbledegook (sorry, can't pronounce your name, th3g... thpfft... trffphhfft... ah, forget it
)
I've been told that you can also use Photo-flo, mixed with distilled water, to clean old negatives and slides. I haven't done it myself, but it sounds workable, and loads cheaper than PEC-12.
However, if those negatives are precious, maybe you should bring them to a pro lab to see what they can do ... most of the time, if they think you can do it yourself, they'll advise you what to do.
Hope this helps ... as always, test first on something that's not important until you figure out the best way to do it.
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You gotta know when to hold them, know when to fold them, know when to walk away, know when to run ...
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