What is the Correct Pressure for Spraying?

Various painting mediums are used to paint interior as well as exteriors of your house, office etc. and spray painting is one of them. It is preferred medium by many as it provides much better results as compared to using brush because it gives much smoother finish and takes less time. Having said that, it is imperative to consider various factors if you want smoother finishing with spray paint. One of the important factors that have major impact on the quality of finish is the pressure of spraying. It is important that you have the right amount of pressure through your paint sprayer gun to avoid uneven paint.

Air Pressure Spraying

Using air pressure guns to spray paint is not as easy as it seems. You have to be careful in setting the pressure of the sprayer just right. In case pressure is too little, you will get an orange peel effect, and with too much pressure, you will waste more material than needed due to excessive bounce-back.

Knowing the ideal pressure is the real art of getting smooth finishing using air pressure spraying gun. Usually, spray-gun manufacturers provide the air pressure details along with their product and this will help you to get the optimum pressure. All said, you might still have to be careful while adjusting the dial to the instructed setting as the manufacturers may not have accounted for the stain or finish you are using to spray. There are other factors as well, like how much thinner you have added, length of your hose and temperature variations etc.

To adjust the pressure at the air cap, you will need an air gauge that can be attached to these places. A special cap with a gauge attached is required to measure the optimum pressure needed and that will cost you extra bucks. However, you do not need such kind of expenditure and neither do you need any suggested setting by the manufacturers. All you need a regulator and a simple test to get the optimum setting.

Best way of adjusting to ideal setting, start by setting every control on the gun to the maximum level and then turning the regulator down to the level well below the level it should be. Now test spray on a piece of cardboard or brown paper. This will give you a narrow pattern with relatively large dots around the edges. Continue increasing the pressure by 5 or 10 psi and test spray on the cardboard until you get your desired spray pressure ideal to serve your need.

It is imperative that the whole time, you are holding the gun at the same distance from the cardboard. Best way to do this is by placing the tip of your finger against the target while the tip of yourthumbagainstthe air cap of the sprayer. This distance is the one best to achieve your perfect finish, that is, 8 inches.

Once you reach the point where the pattern does not widen then previous pattern and does not get smaller dots, you have reached the optimum level of pressure. From this setting, reduce a little to previous setting or a little further.

An important thing to note here, is that the pressure should be changed if you change the material or the weather conditions changes. Moreover, this test does not hold valid for turbine-air supplied guns, as you do not get same control of air pressure with it.

Affection of Air Pressure in Painting

Painting using Air pressure paint sprayers ask for the right pressure for flawless finishing and this can be achieved through afore-mentioned test. Here, another factor is important; high pressure vs. low-pressure painting and their effects on the performance of your coating.

Low Pressure

The problem one can face with setting the pressure lower than the manufacturer’s recommendation is poor quality of spraying finish. You can get poor atomization and/or separation of paint particles. This may result in higher film build than needed as particles coming out are too large. Too much film build also results in slowing down of paint dry time, loss of gloss and popping. It may leave the wall textured.

High Pressure

The other extreme of spray painting is setting the sprayer gun at high pressure. If you set the air pressure higher than the recommended level of manufacturers, it can result in low film build, over spraying and poor metallic control. Using the sprayer gun with high-pressure setting, can split the fan pattern where the pattern in the middle is very light while on the edges, they are heavy. This inconsistent spray paint pattern can cause streaking. This may result in wastage of material and you will be compromising on the durability of the paint system as well.

What is the Correct Pressure for Spraying?

Paint Material Type Fluid Tip Size & Pressure
.032″ .8 mm .040″ 1.0 mm .043″ 1.1 mm
Cup PSI (bar) Air Cap PSI (bar) Cup PSI (bar) Air Cap PSI (bar) Cup PSI (bar) Air Cap PSI (bar)
UNDERCOATS
Lacquer Primer Surfacer 5-6 (.34-.41) 3-4 (.2-.27)
2-Component Primer Surfacer 5-9 (.34-.61) 3-5 (.2-.41)
Primer Sealer 5-6 (.34-.41) 3-5 (.2-.34) 5-7 (.34-.48) 4-5 (.27-.34)
Epoxy 5-7 (.34-.48) 4-5 (.27-.34)
COLOUR SYSTEMS
Acrylic Lacquer 4-5 (.27-.34) 3-4 (.2-.27)
Acrylic/Synthetic Enamels 5-8 (.34-.54) 6-8 (.41-.54)
Acrylic Urethane 4-5 (.27-.34) 4-5 (.27-.34) 5-6 (.34-.41) 5-6 (.34-.41)
Base Coat 4-5 (.27-.34) 4-5 (.27-.34)
Polyurethane 5-6 (.34-.41) 5-6 (.34-.41) 5-6 (.34-.41) 5-7 (.34-.48)
Low V.O.C. 6-8 (.41-.54) 6-8 (.41-.54) 6-8 (.41-.54) 7-9 (.48-.61)
Waterborne 4-5 (.27-.34) 4-5 (.27-.34) 4-5 (.27-.34) 5-6 (.34-.41)
CLEAR COATS
Acrylic Lacquer 4-5 (.27-.34) 4-5 (.27-.34) 4-5 (.27-.34) 4-5 (.27-.34)
Acrylic Urethane 4-5 (.27-.34) 4-5 (.27-.34) 4-5 (.27-.34) 5-6 (.34-.41)
High Solids Urethane 6-8 (.41-.54) 6-9 (.41-.61) 6-8 (.41-.54) 7-9 (.48-.61)
  1. Use the next slower solvent temperature in high volume paint booths and in high temperatures if you want to get maximum flow-out without getting orange peel effect.
  2. Apply clear coats and acrylic enamels in two full wet coats as medium wet coats do not contain enough amount of material for optimum flow-out and may give you orange peel effect as well.
  3. Start with a smaller fluid tip size for best results. To increase the rate of paint flow and speed of production, increase cup pressure and use bigger tip size.
  4. As you increase the size of the tip used, atomizing air cap psi may also need to be increased accordingly. For best results, follow the manufacturer’s recommendations about the ratios for reduction and catalyzation. Use viscosity cup whenever needed.
  5. Pressure calibration gauge is also useful for accurate air cap reading.
  6. When you are changing air caps, the air cap pressure may be disturbed so it may require readjustment for best results.
  7. While using heavy-bodied material, you may need to use full open fan setting, full open fluid knob adjustment and lowest air cap pressure which can provide you with optimum atomization.
  8. Separate cup pressure regulator is also available for increasing the rate of application with heavy-bodied materials. It pressurizes the cup without depending upon the air cap setting.

HVLP System

High Volume Low Pressure or HVLP as popularly known is the trending technique of spray painting of our time. HVLP works with the help of two things, air and the solvent. Solvent is the primer/sealer/paint/clear provided by the manufacturer or recommended to add to the paint. The thinner the PSPC or more air you provide at the fluid tip of the sprayer, the more you will get the PSPC broken down. You need perfect balance. Solvent and air should be in perfect amount to provide you with perfect finishing.

In this process, first thing to look at is your compressor and air supply. HVLP gun main feed is high volume of air to function. High Volume here refers to the high concentration of air needed to operate the gun which is around 50 lbs. at the gun. Low Pressure refers to the actual air cap were the air and paint comes out. The cap of HVLP features only 10 lbs. The real key to the functioning of HVLP gun is volume of air CFM- Cubic Feet per Minute that comes out that provides proper atomization.

For HVLP gun, you need smallest fluid tip that will still let you PSPC the required area and keep that particular area wet for long enough to get adjacent area painted with same thickness of coating. If you use 1.0 tip, it will not be optimum for completing the whole job in time but will be good enough to complete one fender. It will slow down the progress way too much and by the time you finish a single panel, it will be too flashed. Therefore, a compromise here is recommended with the tip 1.3 should be used; even 1.5 can get the job done. Most of the tech sheets provided with the product features suggestions for the optimum fluid tip size.

For various materials, you may need yet different sizes of tips. As, for using polyester primer, a big tip sized 2.3 may work. On the other hand, for urethanes and epoxies, the 1.3 or 1.4 will work fine.

Air Cap CFM Rating and Spray Pattern

Air Cap Type Part Number Material Type 3 PSI(.2 bar) 5 PSI(.3 bar) Gun Distance Approximate Fan Size
GuideRing™ w/Black Ring w/Gold Ring 7042
7043
Full Range 11 CFM 306 L/min 15 CFM 417 L/min 6″ – 8″ 15-20 cm 12″ – 14″ 30-36 cm
AIR-MISER™ w/Blue Ring 7002 Base & Clear Coat Low to Med. Solids 9 CFM 250 L/min 12 CFM 333 L/min 8″ – 10″ 20-25 cm 8″ – 10″ 20-25 cm
HO™ High Output w/Red Ring 7008 Primer & Sealer Med. to High Solids 12 CFM 333 L/min 16 CFM 444 L/min 6″ – 8″ 15-20 cm 9″ – 11″ 23-28 cm
Primer w/Green Ring 7045 Primer, Primer Surfacer 11 CFM 306 L/min 15 CFM 417 L/min 6″ – 8″ 15-20 cm 12″ – 14″ 30-36 cm A

Here are some popular brands of HVLP guns available in the market and this chart features the recommended settings for various materials that you use for spray painting. They may be adjusted to your liking, which can be decided by running the test before doing the actual painting project. For best atomization, use smaller sized tips at first. Increase the cup pressure or fluid tip size in case you want the job done at faster rate. It should be noted however, that with increase in size of fluid tip, atomizing air cap psi might need to be increased accordingly. It is best to follow the manufacturers’ recommendations in the use of tips and adjusting for the right pressure settings according to the material used.

Required Air Inlet

Desired Air Cap PSI (bar) Required Air Inlet PSI (bar)
Round Air Cap AIR-MISER™ Air Cap High Output Air Cap GuideRing™ Air Cap 65 Offset Air Cap
2 PSI (.14) 7 (.5) 18 (1.2) 25 (1.7) 32 (2.2) 18 (1.2)
3 PSI (.2) 11 (.8) 25 (1.7) 32 (2.2) 39 (2.6) 25 (1.7)
4 PSI (.27) 14 (1.0) 30 (2.0) 40 (2.7) 47 (3.2) 30 (2.0)
5 PSI (.34) 16 (1.1) 36 (2.4) 47 (3.2) 55 (3.7) 36 (2.4)
6 PSI (.41) 19 (1.3) 40 (2.7) 56 (3.8) 63 (4.3) 40 (2.7)
7 PSI (.48) 21 (1.4) 45 (3.1) 62 (4.2) 70 (4.8) 45 (3.1)
8 PSI (.54) 24 (1.6) 50 (3.4) 68 (4.6) 77 (5.2) 50 (3.4)
9 PSI (.61) 26 (1.8) 54 (3.7) 75 (5.1) 83 (5.6) 54 (3.7)
10 PSI (.7) 28 (1.9) 58 (3.9) 80 (5.4) 80 (5.4) 58 (3.9)

This chart has the necessary pressure requirements mapped out for you that will guide you about how to use the regulator to get required adjustment on the dial. These conversions provided are for all the Atom-miser® air cap types.

These results shared here are obtained using Mattson 5050 Filter Regulator, 2 Milton Quick Connect Couplers and 30 feet of 3/8″ ID B.F. Goodrich hose.

Air Compressors

DIYers are always on the lookout for best HVLP sprayer guns to cater to their hobby as well as provide them with best results. Therefore, if they have a small setup producing 1.5 – 2.0 horsepower air tank, they are good to go. The only glitch they might face is condensation. Make sure you have at least two water filters; one 10-15 feet from your tank and the other one at your gun neck to keep the water out of your lines.

In case of smaller compressor, you most probably be using a 110 V outlet and the max HP a 110 can put out safely without it kicking off your breakers is 2 HP.

For air Compressors for the HardCoreDIYers, a 50-100 gallon air compressor size having capacity for 189-378 litrewithhorse power rating of 4-6 HP give ample capacity to fulfil their needs. This will be using 240 VAC wiring system of course and one should be prepared to arrange for such kind of power needs as it takes as much power as does home dryer or central AC system.

Pressure setting fares high in perfect finish using both small and large industrial compressors as they are most basic and important setting. Small compressors come with a pressure switch while for the large industrial compressors, they have central compressor controller. It is highly recommended that the pressure setting should be as low as possible for best results.

A compressor always comes with two pressure set points: the cut-in and the cutout set point. They are the points at which the compressor starts and stops. At cutout pressure, the compressor will stop and will starts again once the pressure drops to the cut-in pressure level. This implies that cut-in is lower than the cut-out pressure. The difference between both is called pressure band or pressure differential.

Pressure differential of at least 1bar or 14 psi is standard. If the difference between both is too small, the compressor will repeatedly starts and stops quickly which will take toll on the motor of the compressor and may damage the pressure switch. It is best to keep the cut-in pressure at 6.1 bar at the minimum, which means the cut-out pressure will be 7.1 bar. Keeping the bar high is recommended practice for optimum performance of the compressor.

Tips for setting correct pressure settings

When you are using sprayer gun that uses pressure, compressor that is for operating, you know that there are various settings for the pressure that you can use. The right amount of pressure setting is going to get perfect results you are looking for.

So how you can determine the ideal pressure depends upon various factors. Some spray-guns come with the ideal pressure usage chart with the product itself and that can help you in setting your gun to the right dial. Too little pressure or too high can ruin your wall, as well as your mood.
There are various factors as well that may be overlooked while preparing these charts. They may include the actual stain or how much thinner the paint is, the length of the hose you are using and outside temperature variations. You do not need any kind of gauges to measure these variations as you can figure out ideal air pressure for your gun and for the finishing through just a regulator and the test aforementioned in the article. It will not only give you fairly good idea about how and what is to be done to get flawless finishing, but also gets to save lots of material.

Prepare Carefully

Following the procedure is important here.

  • Start the compressor and let it run until the pressure built up to cut-out level.
  • Slowly let some air escape for the open drain. Note down the pressure level.
  • Adjust the cut-in pressure you just jotted down, with a big sheet screw. Turn it clockwise to increase the cut-in pressure.
  • Close the drain valve and compressor will only run until it reaches the cut-out pressure.

The process is repeated until you reach the right pressure.

Similarly, the cut-out procedure is almost the same as cut-in process.

  • Note down the cut-in pressure at which machine stopped when you last time operated it.
  • Differential pressure should be adjusted accordingly. Clockwise to increase and count-clockwise is used to decrease the maximum pressure settings.
  • Open the drain valve and wait until the pressure level drops down, until its low enough to start the compressor.

In addition, in case you still are facing problems in setting the right pressure in your paint sprayer gun, here is a link that can provide you best answers to your queries.

Conclusion

Correct pressure spraying techniques need mastering and they can be achieved through experience more than reading the manual. It is not a rocket science either and can be learnt by anyone who is interested in using sprayer guns for his or her painting projects. A few tips and tricks can be learned in order to produce optimum results. Make sure you have assembled the gun correctly and the paints you are using are in compliance with the pressure gun, otherwise you cannot ensure smooth finishing at the end. Right kind of viscosity of the material is also a necessity that cannot be overlooked. These few tips and tricks may help you a long way, especially if you are a DIYer.

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