We all like a perfect finish, whether it is spray painting a car, the inside wall of a house, or a garden fence. Despite the most fastidious preparation the end result can resemble a pewter jug - the dreaded "orange peel" effect - if the paint application is not spot on. It is imperative, for a flawless finish, that the compressor can deliver the operating pressure (psi = pounds per square inch) and air volume (cfm = cubic feet per minute) required by the spray gun of choice. Additionally, the amount of pressure drop and volume loss, two of the main causes of imperfect coverage, can be minimised by careful set up of the air compressor and spray system components...
The Hose - How Narrow is the Air Hose?
The hose between the spray gun and compressor/fluid source can be a source of pressure drop if too narrow or too long. A smaller inside diameter (ID) of air hose means less air gets through the line and consequently less air pressure is delivered to the gun. Using a 5/16 inch (7.9 mm) hose means less pressure at the gun than when using a wider diameter 3/8 (9.5 mm) hose.
As a general rule, it is advisable to use a hose with an ID of more than 1/4 inch for all spray guns. HVLP (High Volume Low Pressure) spray guns have larger volume requirements so to ensure maximum air available to the spray gun, a hose larger than 5/16 inch ID is required. However, bigger is not always better as the length of the hose also needs to be taken into consideration.
The Hose - Is the Air Hose too long?
It is important to realise that as the compressed air travels through the hose, the pressure decreases with increased distance from the compressor. Therefore, it is best to use a combination of hose length and ID that reduces the restriction of the air supply.
Quick Disconnect Fittings (QD's)
Although convenient for quick and easy removal of hoses, air connectors are a potential source of loss of air pressure, depending on the number and design quality.
How Many QD's Are There?
A large number of air connectors can be expensive in terms of pressure drop. And are they really necessary? Not really, especially if the air hose is only removed from the gun once or twice a day. Similarly, a quick disconnect attaching the hose the the wall unit may used as infrequently as once a week, yet is contributing to the pressure drop constantly.
Are The QD's Too Restrictive?
Depending on the design and the pressure and volume requirements, the pressure drop from a QD can vary between 1 and 25 psi. It is important to choose a QD design carefully so that the least restrictive units are used. One feature that is indicative of a large pressure drop is a small aperture in the female portion of the QD. If it is small there will be a larger restriction on the air travelling to the spray gun. An opening of 0.196 inches would, for example, provide an area 2.4 times smaller than the area of an aperture of 0.305 inches. Using "high flow" QD's with the biggest diameter practical will ensure minimal pressure loss.
Is the Air Regulator High Flow and of Good Quality?
Air regulators provide precise adjustment of the airflow air pressure for consistent results. However, their design can also restrict air flow and create excessive pressure drop. There are two types of regulator - wall-mounted or gun-mounted. Typically, the larger wall-mounted regulators provide a constant air pressure to the gun and they minimise pressure drop, because the air passage is larger (than in the smaller gun-mounted regulators). The air regulator should be capable of flowing enough air for the spray gun and have a specified capacity greater than the spray gun demand.
One way to determine if the air regulator being used is too restrictive is is to read the gauge on the regulator when there is no paint flowing (static) and when the spray gun is being used (dynamic). The difference between the two readings should 5 psi or less. It is also worth noting that although the air pressure is regulated there may be fluctuations within the system. This can happen if an air-adjustment valve-type restrictor is used as the air will exit the air cap with an initial burst of pressure, which then levels off to the adjusted pressure but may then vary because of changes in system pressure.
What is the Performance Specification of the Air Compressor?
The requirements of the compressor are that it needs to supply sufficient, clean, dry air. It needs to be of the correct size to facilitate the air pressure and volume requirements of the whole system. This is of particular importance considering the previously mentioned restrictions of Qd'ss, narrow hose ID, and alternative air tools. For example, if a compressor provides, for example, 14.7 cfm @125 psi, it is evident that the air supply shouldn't be a problem for a 45 psi gun that requires 13.2 cfm at the gun. Even taking into consideration the drop off in cfm and psi caused by the the restrictions of QD's, small hose diameter, and other air tools, the gun will be capable of optimum performance.
In addition, because this compressor is delivering its cfm @ 125 PSI, and the gun only uses 45 psi, there is plenty of air to spare - because cfm delivered by an air compressor goes DOWN as psi goes up and goes UP as psi goes down.However, if a gun requires 15.8 cfm, there is NO way to get a fine finish from the gun. So if this is the compressor of choice it is necessary to look for a gun that requires less volume of air. The compressor must also provide clean (oil free) and dry (water free) air. The air tank must be drained of any residual water and the compressor filters (that remove dirt, oil and water) checked to determine that they are both clean and functional. It is also advisable to have a final air filter installed at the gun itself.
What About HVLP?
Traditional air spray guns can be thought of as HPLV - high pressure, low volume. Alternatively, the newer HVLP (High Volume Low Pressure) spray guns atomise coatings by employing a high volume of air at low air cap pressure, typically in the 1 - 10 psi range compared to around 45 psi for conventional methods. This means the sprayed material has less velocity so is less likely to "bounce back". HVLP also produced a softer spray, reduces material waste and increases transfer efficiency. However, because they require more cfm than conventional sprayers they need a larger compressor. Like conventional spraying, HVLP spray requires adequate air pressure and volume at the air cap to produce the best results. Consequently, the same factors for minimising pressure drop and volume loss can be considered. In addition, using an air cap test kit with HVLP spray gun systems is useful to verify the air cap pressure setting. This can help ensure there is sufficient air to atomise the material being sprayed and compliance with environmental regulations.
In conclusion, whether using a conventional or HVLP system, the set up of the air compressor for spray guns and painting is crucial for the flawless finish required. With the correct sized compressor, atomisation pressure of the spray gun and set up of the system components - there is nothing preventing a perfect paint job every time.