My spray gun started sputtering a bit the other day. First time ever. So I cleaned it better than I ever have. When I was finished, I put some solvent in the paint cup and everything seemed good. I saw that when I released the trigger however, some bubbles appeared in the cup, but not during spraying. Is this normal? You can imagine that I usually am not looking in the paint cup when spraying so I never noticed this behavior. I figure someone here has to know.