Adjusting Spray Gun Atomizing Air Pressure
Here's my take on setting the proper basic air pressure for a spray gun:
The paint technical data sheets usually give the recommended pressure "at the gun". For non-HVLP guns, that pressure is usually between 40 and 60 psig. For HVLP guns it is always 10 psig or less.
There is a major difference in how this pressure is interpreted for non-HVLP vs HVLP guns. For Non-HVLP guns this pressure is that which should exist at the air inlet connection to the gun while the trigger is pulled with the air adjustment wide open. The 10 psig maximum for HVLP guns is an environmental legal requirement in order to comply with required HVLP transfer efficiency. It is the maximum pressure that can legally be applied "at the air cap" and is the pressure existing just behind the air cap and immediately prior to the small orifices in the cap. This is not the same thing as the inlet to the gun. It will always be less than at the inlet, sometimes a lot less. How much less depends on the design of the internal air passages in the gun.
Unless you have specialized test equipment, you will not be able to measure this pressure "at the air cap". For HVLP guns, an equivalent pressure "at the gun inlet" that will result in a maximum 10 psig "at the air cap" should be given in the documentation that came with the gun; or stamped on the gun handle itself. My HVLP spot gun has this inlet pressure stamped on the handle as 29 psig max. This is interpreted to mean when I have 29 psig at the inlet to the gun, with the trigger pulled, the cap pressure will be no more than 10 psig and the legal requirement for HVLP will be met. It does not mean that the gun will be harmed if you increase the pressure (within reason of course)...only that you may be breaking some law, depending on your situation and location.
HVLP gun inlet pressures required to keep the cap pressure at a maximum of 10 psig are different, depending on gun manufacturers. I think about 30 psig is typical but I have read about pressures as low as 15 and as high as 50 psig. Because these pressures are different with different guns, the paint manufacturers cannot give an HVLP inlet pressure in their paint technical data sheets. They can only say a maximum of 10 psig for HVLP, in order to comply with legal requirements. A good tech sheet will tie it down to 10 psig "at the cap". A poor tech sheet will just say 10 psig and leave it up to the reader to know what it means. For HVLP, you need to get the correct gun pressure from the gun manufacturer and not the paint manufacturer.
In order to properly atomize the paint you should always measure the pressure right at the gun inlet and adjust your regulator to give the proper amount. I have pressure gages installed at the inlet to my guns and simply leave them in place. If you have a wall-mounted or compressor-mounted regulator you don't need another regulator (or needle restrictor valve) at the gun, just a pressure gage. It should not be one of these little dinky things where you can't distinguish anything finer than 5 psig. Use a 2" gage marked in 2 psig increments so you can easily read it. For non-HVLP a 0-100 psig gage would be appropriate. For a HVLP gun a 0-60 one would be a better choice. Here's what my HVLP spot gun looks like with gage attached:
It makes a difference, sometimes a big difference to measure this pressure right at the gun rather than back at some wall-mounted or compressor-mounted regulator. For example, I typically use about 50 feet of 3/8" ID air hose supplied from a wall-mounted regulator. For my spot gun I also use an 11 foot length of 1/4" ID whip hose just prior to the gun to permit easier handling. In this configuration there are two 3/8" quick couplers and two 1/4" quick couplers between the wall regulator and the gun. My HVLP spot gun consumes a low 4.8 cfm per the manufacturer (at 29 psig inlet). When I use this gun, I adjust the wall regulator to give about 30 psig on the gage at the gun, with the trigger pulled. The wall regulator will read 39 psig. That is a 9 psig drop from the regulator to the gun. This does not include pressure drops for equipment upstream of the regulator, such as my air filters, or the regulator itself. With my full size non-HVLP gun, the difference is a lot greater. It depends on which air cap is being used but typically the volume requirement is 13 cfm when there is 50 psig at the gun inlet. Fifty psig inlet pressure is typical for this non-HVLP gun. I do not use the small 1/4" whip hose that I use with my spot gun. The configuration is therefore 50 feet of 3/8" hose, two 3/8" quick couplers and one 1/4" quick coupler. This arrangement will require 67 psig at the regulator to achieve 50 psig at the gun inlet. That's a 17 psig drop from regulator to gun.
Such pressure drops are caused by frictional effects in the hose and resistances in fittings, particularly quick coupler assemblies. They can be minimized but never avoided. If you set the pressure using a gage "at the gun inlet", you will be compensating for whatever drops you may have in your own system.
Remember that it does not matter what a gage back at your regulator says. It is the pressure at the actual inlet to the gun that is important. It must be measured and adjusted while the trigger is pulled. There does not have to be any paint in the gun at the time but the air adjustment on the gun should be wide open for a maximum pattern. Adjusting the pressure this way can make a significant difference in how well the paint is atomized by the gun and consequently how well it flows out on the surface.
There may be other ways to approach getting the correct basic air pressure for paint atomization but this is how I have done it successfully. I can't say that I have ever found the need to increase air pressure beyond recommended ranges, although I do prefer to use the middle to upper end if a range is given. There are also procedures to fine-tune the air pressure based on pattern tests but I haven't found that extra effort to be necessary to get results I consider satisfactory. This is applicable to guns supplied from air compressors only. I have no experience with turbine-supplied guns which are very different.