I've never used fixer with hardener on my negatives. I'm wondering if there are any disadvantages to using it?
It takes a little longer to wash. Read the sticky post about film washing for a comparison.
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Yes, washing takes longer...
and retouching is more difficult.
Can be a bit less archival as it's harder to remove residual silver, and it's far less environmentally friendly in terms of water usage.
Except for some poorly hardened films and tropical, or high temperature processing there's no benefits of using a hardener for films.
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It's useful for Efke films (& of course the Adox equivalents) as the emulsions are so delicate.