Sandra, I fully agree with you about all little details that make the salt working or not working with watercolours, e.g. overall humidity, painting with more water and waiting a bit before adding the salt. It works differently on different papers. Some pigments also go under the salt and make the final impression flat, without creating any pattern.
I prefer using the salt with certain pigments that are likely to give a granulated effect on any paper. When the painting is completely dry (note it is very important to have it completely dry), I gently go over it with transparent layer of any colour that I want to finish with (glazing!).
@mmcbuck The best way to try the salt out is to take different papers and different granulating pigments, and to combine pigments vs. papers vs. time. Here, the time dimension means applying the salt 2 minutes after adding the layer of pigments, or 5 minutes, or just before the layer gets dry. In that way, you will get the results that can be easily compared and used to extract the wanted effects. From my experience, the salt shouldn’t be touched before the painting get fully dry.