I have an inexpensive gun I was using to spray self etching primer. It sprayed one cup full and it worked just fine. I then loaded up the cup again and it started sputtering as it sprayed. I quit painting. I took apart the gun and cleaned it completely. I then tried spraying straight laquer thinner and it still sputtered. I tried adjusting the air pressure, the fan pattern and the fluid control knob. No adjustment made any difference. While spraying I noticed air bubbling up in the cup. Is the gun doomed or what might be the problem?