Are these nibs showing up on the underside of the panels, And areas where dust could not fall down in to it?
80% of the dirt in a paint job, you bring to the surface, on your clothes, in the paint gun etc etc.
Sometimes "cleaning" a paint gun, dont mean its really clean. The residue left behind by some thinners will cause some materials to ball up like that, usually that stuff only happens in a small area tho, same as the dirt on your clothes.
More likely your mixture is not right for the conditions, too slow or too fast reducer, wrong hardener or too little reducer etc, basically it looks to me as though the primer is drying on the aircap, and being blown in to the wet paint, a chunk at a time.
Do you have a REALLY good water trap and and particle trap? The **** comming out of even a brand new aircompressor will blow your mind, Chuncks of rust, oil, and bits of sealant from the lines, and pump itself.