I also put sea salt (no Iodine) in Rodinal at times to kill some of the graininess with some films. It works fine. I think I use 30g/liter IIRC. It seems to lose some speed though which makes sense. From what I remember reading the salt acts on the exposed silver somehow to limit infectious development giving the finer grain. Some of the old fine grain formulas used salt.
30 g/l NaCl was supposedly used in Microdol for fine grain at the expense of emulsion speed. Note that Microdol X used an extra compound to avoid dichroic fog, so it would be interesting to know whether you ever ran into that issue with modern film.
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