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I bought the paint for my wife's '56. It is PPG DBC base coat. The tech sheet says for spraying: 8-10 PSI at cap for HVLP gun, 35-45 PSI for conventional gun. I have a HVLP gun with a pressure regulator at the inlet. How do I figure the pressure at the cap?
The pressures you normally see in "tech sheets" are to set the air pressures to meet EPA regulations. Normally requires a bit more for optimal spraying.. Here's a good reference....
http://www.spiuserforum.com/index.ph...84/#post-54102
8-10 psi "at the cap" is what's technically required to have a spray gun be called "HVLP". Thing is, you need to know what pressure at the inlet should be.
I have a Devilbiss gun that wants 23 psi at the inlet to have 10 psi at the cap. I also have a Sharpe gun that wants 50 psi at the inlet to have 10 psi at the cap. So try to find the specs on the gun you have. And remember that these are only starting recommendations, your gun may spray better with a bit more pressure, or maybe a bit less (unlikely). Also remember that pressure gauges aren't that accurate, find out what your gun likes with the gauge and pressure regulator you use.
Maybe I am in left field here but I think the OP is wanting to know "how" to figure out that pressure. What I have been told, read and do is to put about 50 PSI to the gun from the compressor, then hold the trigger open on the spray gun about 3/4 and read/adjust the pressure at the gun inlet with a second regulator/pressure gauge that's inline. But as most have already mentioned, you usually can't spray the paint very well at that spec sheet setting. Best is to trial and error test the spray pattern until you have it where you want it.
Thing is, that gun that calls for 23 psi isn't going to spray worth a crap at 50 psi. You might as well get a close starting point. That Devillbiss gun seems to work at 25 psi on the gauge I have.
The majority of guns do need around 50 psi. My Devillbiss is an exception. It's a primer gun, but it sprays that well.
One other point, I've only see one gun ever that had a way to measure pressure at the air cap. Even then, it was a piece not used when spraying. You had to put the special cap on to measure, and the regular cap back on to spray. It was a gun for a turbine setup.
Thanks to everyone for the replies. Now I have the info i needed to start spraying.