Hi Natascha, about the ink consistency: I meant not like gouache from the tube , but when it is already mixed. So maybe that's confusing! But surely the ink I have is less watery than for example Zeichentusche. Hmm. Maybe a foto to make sure we are talking about the same product?
About the Gummi A., that sounds strange. Maybe it's already off? Mine is definitely not grainy, and I also have the Schmincke. I add it with a pipettes (just some drops!) to fountain pen ink.
I am really not sure if the g.a. and schellack are chemically compatible. Or are the grains already in the g.a. bottle before you mix it?
Regarding mixing, I use toothpicks to stir inks.
Hope that helps!