What is Wet Paint?
A spray application, which is the most common paint application method, is based on the application of the paint by spraying to atomize in small droplets. It has a wide range of uses due to its variety and ease of application. Obtaining surfaces with uniform thickness distribution and well-spreading caused this method to diversify and become widespread in industrial paint applications. However, the practitioner must use masks and a similar personal protective equipment. In addition, during the application, the amount of paint spread on the environment, in other words, the paint transfer efficiency varies according to each spray application.
1. Conventional (Air) Spray Applications
In the conventional spray application method, the liquid paint exits from the narrow gun nozzle through an air spray gun, mixes with compressed air and is sprayed in very small droplets.
Paint can be supplied to the gun in one of three different ways. Different gun structures are designed for each method.
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Top chamber gun: Paint is fed to the gun by gravity from a chamber mounted on the gun (Figure-2). It is very suitable for small jobs with frequent changes of the color. |
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Bottom chamber gun: The paint is fed to the gun with the suction effect from a reservoir mounted under the gun (Figure-3). It is well suited for small quantities and frequent discoloration, such as gravity guns. |
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Internal Mixing: As shown in Figure-4, the meeting of air and paint takes place just before the nozzle of the gun. It provides the use of less and lower pressure air in the internal mixture. It creates a wider beam, provides maximum film thickness and less overspray formation is observed. The needle and nozzle should be disassembled and cleaned after each painting. Therefore, it is more suitable for the use of slow drying materials. |
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External Mixture: In such applications, air is sprayed from the air ducts on the gun head, encounters with the paint coming out of the nozzle of the gun and creates a mixture (Figure-5). It is a widely used method. A better atomization can be achieved with external mixture. The beam can be controlled better and can be used in bottom chamber guns due to its high pressure. It can be preferred in applications that require fast drying and high quality surface. It is easier to clean. |
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When the air is pressurized, the moisture it contains can condense and these water particles can reach the surface of the paint and cause a surface defect we call bubbles. In addition, since it is possible for the compressor oil to mix into the air, "water and oil traps" are installed between the compressor and the gun. There are two adjusting valve heads at the back and one at the bottom of the gun. The upper one of the valves on the rear is used to adjust the size of the paint jet, and the other is used to adjust the flow rate of the paint to be sprayed from the nozzle. The adjustment valve at the bottom is used to adjust the amount of air fed to the air ducts (Figure-6). |
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While adjusting the paint flow and air flow with valves, there is no change in pressure. However, after the size-adjusted paint jet leaves the gun, it encounters the air flow of the outside environment. Therefore, the beam expected to be circular turns into an elliptical structure providing better efficiency (Figure-7). |
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The breakdown of the paint into very small droplets, also called atomization, is determined by the following variables.
• Application viscosity of the paint (as the paint viscosity increases, the drops get larger),
• Diameter of the nozzle hole (the smaller the diameter of the nozzle hole, the smaller the drops),
• The ratio of paint pressure to air pressure (as this ratio gets smaller, the drops get smaller),
• Surface tension of the paint (the drops get smaller as the surface tension decreases),
• Paint pressure dragging the paint to the nozzle hole (the drops get smaller as the paint pressure increases, provided that the paint pressure / air pressure ratio remains constant),
Good atomization means that the droplets become as small as possible. Droplet diameters between 20 µm and 50 µm can be reached in air spray applications. This is an atomization that allows a very good paint dispersion. At this point, rough paint application can be mentioned as an exceptional application. In rough paint applications, it is aimed to obtain a rough surface with indented protrusions that do not spread well as a result of spraying the paint in large droplets by reducing the air pressure and selecting large diameter spray nozzles. It is especially preferred that some machines and auto trunk interiors are painted to have this appearance.
In air spray applications, 600 volumes of air are used for 1 volume of paint. Therefore, the material sprayed is a paint-air mixture that contains plenty of air. This excessive amount of air in the mixture causes three important consequences.
a) In air spray, the material goes towards the target as a cloud with indistinct boundaries. Even if the application is made directly on the surface, some of the paint goes beyond the surface.
b) The paint particles, which reach very high pressure with the air, hit the surface with great speed and a significant part of the hitting paint splashes back and spreads around (Figure-8).
c) The air exiting with a high pressure of 2.5 - 4 bars and the paint coming out with less pressure than the air reach the surface to be applied at very high speeds. As the air hits the surface and rotates, some air clouds accumulate on the surface and create the effect we call air cushion. Some of the paint coming out of the gun hits this air cushion before reaching the surface and they tend to bounce back. This is called the "rebound effect" (Figure-9). The returning paint particles (also known as overspray dusts) spread to the environment.
For these two reasons, paint losses are high and transfer efficiency is low in air spray applications. The transfer efficiency may decrease to -50% depending on the shape and size of the object to be painted, and it can go up to p on large surfaces. Due to the low transfer efficiency, the amount of paint required to cover the surface of the paint and the amount of solvent released into the environment increase. The high amount of paint powder and solvent (VOC value) emitted to the environment creates unhealthy conditions and does not allow applications to be carried out in open areas. Therefore, applications should be made in paint booths. Along with the booths, facilities where paint dust, paint sludge and dry paint can be filtered should be established. In addition, the person who will do the application must apply with the necessary personal protective equipment.
Advantages
• It can be used on all kinds of surfaces.
• It provides good atomization.
• Its application area is wide. It can be used in various areas where tons of paint are used by selecting the appropriate gun.
• Provides good surface quality.
• It can be used with bottom reservoir, top reservoir and pressure vessel. (Systems that allow the flow of paint with the help of pressure vessels are generally preferred for large painting.)
• It provides the opportunity to change the pressure and flow rate of the air and paint and the mixing ratio of the beam.
• It gives good results especially in effect color applications where color sensitivity is important. It is the most ideal application method for effect paints.
• Equipment cost is relatively lower. Disadvantages
• Too much paint dust and solvent are scattered around during the application. Therefore, it requires the installation of special application cabinets. Cabin filters should be changed frequently.
• The cleanliness of the gun is also important. Cleaning processes require precision and time.
• Transfer efficiency of the paint is low.
2. High Volume Low Pressure (HVLP) Spray Applications
Achieving high atomization and good surface quality in conventional air spray applications increases the prevalence of this application method. However, the underlying problems such as low transfer efficiency, high cost of paint reflected to the user and high environmental pollution led to the search for other methods.
During these searches, atomization and the surface quality to be obtained are important. The following factors are effective in atomization quality:
• the viscosity of the paint (high viscosity = large particles) - depending on the paint
• surface tension of the paint (low surface tension = small particles) - dependent on the paint
• air pressure (high air pressure = small particle) - attached to gun
• nozzle diameter (small nozzle diameter = small particle) - depends on gun
• paint pressure (large pressure = small particle) - depends on gun
• air volume (high air volume = small particle) - depends on gun
The viscosity and surface tension of the paint are not related to the choice of gun. These factors depend on the formulation of the paint. The increase in air pressure, which is a variable dependent on the gun, is a factor that increases the atomization quality. The effect of the nozzle diameter is less than the pressure and it can be used by changing it depending on the viscosity. The pressure of the paint is either constant in the feed containers or in the others (bottom and top chambers) again depends on the pressure of the air. Therefore, in order to achieve good atomization at low pressure, it is necessary to increase the air volume. In this way, the high volume of air entering between the paint droplets expands with its exit to the outside environment and breaks the paint droplets into smaller droplets. The working principle of high volume low pressure (HVLP) air spray guns is based on these factors.
With the development of HVLP guns it has been possible to increase their transfer efficiency up to twice. HVLP guns are designed in detail to allow abundant air supply of gun air ducts. The paint is sprayed at high flow rate but low pressure (with 0.2-0.7 atm air in HVLP systems instead of 2.5-5.5 atm in conventional systems). The low air pressure reduces the rebound effect too much, leading to an increase in transfer efficiency. The widespread use of HVLP systems is gradually increasing, especially in the field of auto refinish paints. In HVLP applications, an atomization and spreading quality close to that obtained in conventional applications can be achieved.
The paint particles atomized with the HVLP gun are slightly larger than the atomization in conventional guns. Therefore, the obtained surface quality is expected to be worse than conventional applications. However, with recent studies, levels close to the surface quality applied with air systems can be reached. However, the low air pressure prevents the paint particles from hitting the surface rapidly and bouncing (Figure-10). This increases the efficiency of the application by 70-75%, thus leading to savings in paint consumption.
Compared to conventional spray applications, in applications with HVLP guns, there is a decrease in the amount of overspray dust emitted to the environment and the amount of waste causing environmental pollution. Nevertheless, several restrictions have been imposed in some countries to prevent overspray dust during the use of HVLP guns that may cause environmental pollution. For example; In some states of the USA and some European countries, the use of guns other than HVLP with a pressure of 0.7 bar in the air cap is prohibited. In addition, it is required to use HVLP guns that provide 65% or higher transfer efficiency at pressure values of 0.7 bar and below this value in Southern California, USA.
In HVLP and conventional guns, the inlet pressure of the air to the gun and the outlet pressure in the air cap are given in Table-1. When the values in the table are examined, the transfer efficiency of HVLP guns is higher than conventional guns.
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Table – 1 Pressures and efficiency in HVLP and Conventional guns. |
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Inlet Pressure |
Outlet Pressure |
Transfer Efficiency |
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Conventional Gun |
3 – 5 bar |
2,5 – 4 bar |
30 – 50 |
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HVLP Gun |
2 – 2,2 bar |
0,7 bar |
70 - 75 |
When the application methods of these two methods are examined, the HVLP gun should be applied closer to the surface and a little slower than conventional guns (Table-2).
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Table – 2 Application parameters in HVLP and Conventional guns |
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Distance to Surface |
Application |
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Conventional Gun |
20 – 22 cm |
Fast |
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HVLP Gun |
10 – 15 cm |
Slow |
In powder coat applications, it is not recommended to reduce the pressure as in a conventional gun, because the atomization quality will be adversely affected when the pressure drops too much. Instead, it would be more appropriate to increase the application distance.
Power of compressors used in HVLP guns; Although the manufacturer varies according to gun quality, nozzle diameter and air cap, it is approximately between the values specified in Table-3. This much power requires higher electricity consumption than conventional applications, but with the HVLP application, savings in paint consumption are also provided.
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Table – 3 Air consumption and required compressor power in HVLP and Conventional guns |
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Air Consumption |
Compressor Power |
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Conventional Gun |
275-350 L / min 9,7 – 12,4 cfm |
1,6 – 2,1 Kw 2,2 – 2,8 HP |
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HVLP Gun |
410 – 460 L / min 14,5 – 16,5 cfm |
2,4 – 2,7 Kw 3,2 – 3,6 HP |
Since the air volume used in HVLP guns is high, the inner diameter of the hose to be used should be 0.9 cm (the conventional gun is also around 0.8 cm). Therefore, when switching from conventional gun to HVLP gun, a user has to change gun, compressor and hose.
In HVLP applications, as in conventional applications, pressure coated, bottom chamber and top chamber gun types can be used. Very good transfer efficiencies can not be obtained with bottom chamber HVLP guns. The reason for this is that the pressure of the air carrying the paint to the surface is low, sufficient suction can not occur and as a result, the paint can not be sprayed efficiently. Since gravity is used in top chamber guns, the transfer efficiency is better. The best efficiency is provided with pressure fed guns.
3. Low Volume Low Pressure (LVLP) Spray Applications
In HVLP guns, the atomization quality, which decreased with the decreasing pressure, was increased with high air volume. However, the fact that HVLP spray applications require the use of a stronger compressor due to high volume air consumption makes it seem a difficult type of application for users who are accustomed to application with a conventional gun. Expectations for both a reduction in air volume and an increase in transfer efficiency have guided the development of LVLP guns. With the development of LVLP guns, the exit pressure of the air from the nozzle of the gun is kept somewhat higher than HVLP guns, thus reducing the required air volume. LVLP guns appear to have the same or even slightly less air consumption than a conventional gun. In addition, in these guns, the paint and air are mixed in some amount of the gun to reduce the required air volume, but this solution is not a very
applicable method for spray gun manufacturers.
As mentioned before, the use of HVLP guns with an air cap (horn) pressure of 0.7 bar is mandatory in some states of the USA, and in other states and Europe, it is required to use low pressure guns with a transfer efficiency higher than 65%. However, there is no restriction on the air cap pressure. Thereupon, spray gun manufacturers have produced guns that operate at low pressure compared to conventional but do not worry about the "requirement of 0.7 bar air cap pressure" imposed in the use of HVLP guns and that can be used by increasing the pressure slightly when necessary. For example, SATA has put "LVLP" guns into production under the name of "RP-Reduced Pressure". In RP guns produced by SATA, the inlet pressure is around 2-2.5 bar and a pressure of 0.7 bar or slightly higher is obtained in the air cap. Transfer efficiencies close to HVLP guns are obtained. At the same time, the air cap pressure being slightly higher compared to HVLP guns increases the application speed (Figure-11). Similarly, DeVilbiss introduced the GTI pistols as "Compatible Pistols". Thanks to the technology used in these models, the atomization quality is also increased.
When Table-4 is examined, it is seen that the transfer efficiencies of HVLP and LVLP guns are almost the same, but they are higher than conventional guns.
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Table – 4 Pressures and efficiency in LVLP, HVLP and Conventional guns. |
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Inlet Pressure |
Outlet Pressure |
Transfer Efficiency |
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Conventional Gun |
3 – 5 bar |
2,5 – 4 bar |
30 – 50 |
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HVLP Gun |
2 – 2,2 bar |
0,7 bar |
70 – 75 |
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LVLP Gun |
2-2,5 bar |
≥0,7 bar |
65-70 |
Although the manufacturer, gun quality, nozzle diameter and air cap vary for each LVLP gun type, a typical LVLP gun consumes 260 - 300 L / min (9.2-10.6 cfm) of air and a compressor requires 1.5-1.9 Kw (2.0-2.6 HP) (Table-5). When these results are examined, it is seen that the compressor used for conventional guns will also be sufficient for LVLP guns and a service will not be needed to replace the compressor. However, the expected costs increase, although slightly, due to the use of excess air in HVLP guns, it’s not seen in low air volume LVLP guns. Due to the low volume of used air, there is no need to change the inner diameter of the hose.
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Table – 5 Air consumption and required compressor power in HVLP and Conventional guns |
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Air Consumption |
Compressor Power |
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Conventional Gun |
275-350 L / min 9,7 – 12,4 cfm |
1,6 – 2,1 Kw 2,2 – 2,8 HP |
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HVLP Gun |
410 – 460 L / min 14,5 – 16,5 cfm |
2,4 – 2,7 Kw 3,2 – 3,6 HP |
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LVLP Gun |
260 – 300 L / min 9,2 – 10,6 cfm |
1,5 – 1,9 Kw 2,0 – 2,6 HP |
When the air caps are changed in the guns, the number of holes in the nozzle and the diameter of the holes will change, so the amount of air it will consume changes. For example, DeVilbiss's GTI gun consumes 453 L / min when the 105 numbered air cap is installed at 2 bar inlet pressure, and 269 L / min when the 110 air cap is installed. However, it would not be correct to call the gun HVLP, LVLP or conventional according to the amount of air consumed because the pressure in the air cap changes. Air pressure in the air cap must be considered for this type of naming. Air pressure in the air cap is 2.5-4 bar in conventional guns; 0.7 bar in HVLP guns; It is expected to be 0.7-0.8 bar in LVLP (Table-4).
4. Airless Spray Applications
In airless spray applications, the paint is pressurized between 5 and 35 atm, and it is sprayed from the nozzle of the airless spray gun. As the paint jet coming out of the nozzle as a thin curtain and at high speed hits the stagnant air molecules outside the gun, it tears and turns into droplets. The size of the droplets will be smaller, when;
• As the dye outlet speed increases,
• as the paint pressure increases,
• the lower the paint viscosity is and
• As the surface tension of the paint decreases.
The size of the beam depends on the hole size and shape. However, in airless spray applications, the atomization quality of airless applications can not be reached. While the diameter of the paint droplets sprayed from the nozzle of the gun reaches 20-50 µm in air spray applications, this value is in the range of 70-150 µm in airless spray applications. On the other hand, the decrease of paint droplets from the center of the paint jet in air applications towards the edges allows for smooth transitions between applications. Therefore, it is easier to apply homogeneous thicknesses. In airless spray applications, they don’t give this opportunity because they do not contain air. It is more difficult to apply uniform thickness with airless spray applications.
In terms of application speed, airless spray applications provide faster solution. However, on uneven and complex surfaces, it may cause paint to fall thick and bleed.
In airless spray applications, since there is no compressed air accompanying the paint droplets, the amount of solvent flying before the paint reaches the surface is less. This means that the paint reaches the surface in a drier state. For this reason, types that evaporate faster should be used both in the selection of the solvents in the paint formula and the application thinner.
Compared to air spray applications, the effect of bounce back and as a result of the paint particles spread to the environment is less in airless spray applications. Hence, the transfer efficiency is relatively higher.
There are also situations where air-assisted but airless systems are used. In these systems, the pressure is higher than the air pressure in air systems and less than the airless system. Atomization in such systems is much better than airless systems.
5. Electrostatic Spray Applications
Electrostatic spray application, which can be used with air, airless or other systems, is preferred for applications on complex shaped metal surfaces and powder paint applications where overspray should be less. In electrostatic spray applications, the air around the point where the paint is sprayed is ionized by applying an electrical voltage of 50-125 kV. The electrons released as a result of the ionization of the air attach to the surfaces of the dye particles passing through this region and charge them with a negative (-) charge. Before the application, the surface which has to be painted is grounded and loaded with a positive (+) load. Thus, an electrostatic field is created around the surface which will be painted. In this case, the negatively charged paint particles are enabled to reach the surface with the effect of the electrical attraction force on the object (Figure-12). This electrical attraction greatly reduces the spray losses that may occur due to fallout and rebound effect. It minimizes paint losses especially when painting objects such as pipes, profiles and their backs that cause spray loss.
The electrical conductivity of the paint must be sufficient so that the paint particles reaching the surface can transfer their load to the soil over the surface. In electrostatic applications of solvent paints, high conductivity solvents or additives such as alcohols are added to the product or to the thinner. The widest range suitable for wet paint conductivity is between 0.05-20M. However, a conductivity of 0.5-5 M is preferred for most.
Spray guns using air as a propellant are widely used in electrostatic spray applications. In these applications, the spread level of the air spray is achieved and the spray losses are minimized. In electrostatic spray applications, disc or bell-shaped rotary head application tools are also used, where the paint is broken up into very small droplets by the effect of centrifugal force.
Rotational speeds of 1000 revolutions per minute (1000rpm) and above are common in rotating discs. Rotation speeds between 25000 rpm and 60000 rpm are applied in very high speed bells. It is possible to encounter low gloss problems in paints applied at such high speeds. Therefore, precautions against the tendency of dullness caused by high speed should be taken during the design of the paint.
With the introduction of some organic solvent and making suitable electrical insulations on the application equipment, it is possible to apply the watercolors by electrostatic methods.
On the other hand, surfaces such as non-conductive plastic can be made conductive by first applying a thin layer of conductive primer with normal air spray method. Subsequent coating layers can be made by electrostatic spraying method.
Spray losses can be reduced and extremely well-dispersed paint surfaces can be obtained with electrostatic spray applications made with either a gun or rotating disks or bells. However, this method also has some disadvantages. Especially the recessed parts on the surface may not get enough paint due to a problem called "Faraday cage effect" (Figure-13). High voltage applications cause this effect to be experienced more intensely. As a result, a thicker paint film is obtained on surfaces where the paint can easily reach, and a thinner paint film is obtained on surfaces with a Faraday cage.
The second possible source of trouble is the risk of fire. Most of the solvents used in solvent paints can be explosive at room temperature and below. Electrostatic applications where sparks may occur due to the applied voltage should be set so that the flash point of these solvents is 26 ° C and above.
Advantages
• Among all spray applications, the lowest paint loss and minimum paint dust are obtained in electrostatic applications.
• Transfer efficiency is higher.
• In electrostatic applications, angular, round and indented-protruding metal materials can be coated with equal film thickness.
• Provides a clean working area for the practitioner.
Disadvantages
• Electrostatic spray can only be used for painting conductive materials.
• Although it is possible to reach high film thicknesses, a second layer can not be electrostatically applied since the surface will be isolated after one coat of paint is applied.
• Equipment cost is high.
• Not useful on large surfaces.
• May be dangerous at high temperatures (> 25-30 ºC).
• It may give different appearance in metallic paints compared to a non-electrostatic application.
6. Hot Spray Applications
In hot spray applications, the paint is heated to temperatures between 40ºC and 65ºC, resulting in a decrease in the viscosity of the paint. In this way, application viscosity can be achieved by using less amount of thinner. With the use of low amount of thinner, the amount of solvent emitted to the atmosphere is reduced and thicker paint films can be obtained by applying less layers. On the other hand, the fact that the temperature of the coated surface is relatively lower than that of the paint ensures that the paint cools as the paint hits the surface, increases its viscosity, and consequently the risk of flowing on the surface is greatly reduced.
However, a heating device is required for such an application method. However, the necessity of designing the heating in such a way that it does not cause glare and fire risks limits the use of hot spray applications.
7. Spray Applications with Liquefied Carbon Dioxide
In this application, carbon dioxide (CO2) gas is made supercritical under high pressure and liquefied and dissolves the liquid organic coating material like an organic solvent. In this supercritical phase, the liquefied CO2 reaches a critical temperature of 31.3°C and a critical pressure of 7.4 Mpa. CO2 has solvent characters similar to aromatic hydrocarbons in its supercritical phase. Therefore, it is possible to replace some of the solvent in paint with CO2. Special airless spray guns can be used for this purpose. As soon as the paint + CO2 mixture leaves the gun, the CO2 is transformed into a gas with a vigorous boil and is released into the atmosphere. However, since CO2 gas is not VOC, it does not cause atmospheric pollution. On the other hand, CO2 homogeneously distributed in the atomized paint droplet at the exit from the nozzle of the gun encounters atmospheric pressure here. With the decrease in pressure, CO2 expands rapidly and passes into the gas phase, and a very high quality atomization is achieved by the parts inside the droplet. Therefore, although the application is made with an airless gun, the surface quality obtained is at the level obtained with an air spray gun. Since the application is done with an airless gun eliminates the effect of rebound seen in applications with air gun, the transfer efficiency achieved is close to that obtained in airless applications. In addition, as the CO2 leaves the paint quickly, the viscosity of the paint reaching the surface from the gun increases rapidly, thus the risks of encountering paint defects such as flowing and sagging are minimized.
As a result, it is possible to combine all the positive properties obtained in various spray applications in airless spray applications with liquefied CO2. The problem is the cost of the liquefied CO2 and the delivery device.