I can only be of some help, relating the effect (graininess) from other types of spraying (automotive) since I'm new to AB'ing.
Usually a grainy or sandpaper finish is caused from the atomized paint drying "before" it gets on the object.
You can try getting closer to the object or more paint flow than air flow.
Do a drip test, compare how the polyscale gloss drips compared to a
paint that you have good results with. If they both drip fairly equal,
I would try changing my painting technique (above) before changing the
viscosity of the paint.
Hope someone has a more definitive answer for you.