Have your friend join the forum and post here so we can honestly see if the compressor is running more than 50% of the time which means it isn't catching up to the volume needed.
Well here is my Viber conversation with him
"Well technically you need 4.0 SCFM at 40 PSI to keep the gun running indefinitely.
But the trick is that you can save money on a compressor because weaker compressors can still do the job.
It can still get to the required pressure, it just cant keep it at that pressure level for long.
So you turn the compressor on, let the tank fill up, and then use it until the pressure falls and then you let it fill up again.
I have found that with this compressor, one full tank can usually get me through a coat."
He has the Bostitch pancake compressor I listed in the beginning of the thread...