Basics of Spray Painting

by Paul Crumpler, Pollution Prevention Engineer

A basic spray painting system consists of a conventional spray painting gun, a pressurized paint container, a compressed air system, and flexible tubes connecting it all together. This type of system has a transfer efficiency ranging between 20% and 50%. Improper setup and painting techniques will increase the amount of paint used and cost.

How Conventional Paint Guns Work

A conventional spray gun uses air to atomize paint and to provide a spray or fan pattern. Paint flows through a tube from a paint pot pressurized from 8 to 30 psi. Atomizing air is supplied to the gun tip from 30 to 90 psi. The pressure in the paint pot or fluid pressure pushes the required amount of paint through the paint lines to the gun tip. There, the atomizing air breaks the paint into small particles and propels it to the surface that is being painted. The required fluid pressure is based on delivering a certain amount of paint through the hoses to the paint gun tip. For example, a typical gun might apply 10 ounces of paint per minute with paint supplied through 15 feet of 3/8" hose. Based on the viscosity of the paint, size and length of paint supply hose, and pressure drop through filters, it may take 18 psi to push 10 ounces of paint though the fittings, hose, and gun. A very viscous paint may take 30 psi. Depending on properties of the paint, size of tubing, size of paint line filter, and number of pipe fittings, higher or lower fluid pressure may be required. Paint viscosity will also affect atomizing air pressure.

There are many different types of spray coating equipment. All of them work by atomizing dry or liquid paint and directing the paint to a surface. The other common types of spray application equipment are high volume -- low pressure (HVLP), airless, air assisted airless, and electrostatic enhanced.

Transfer Efficiency and Coverage

Transfer efficiency is simply the theoretical coverage of a paint versus the actual coverage after the paint is applied. Assume that a paint is purchased that has a maximum or theoretical coverage of 300 square feet per gallon with a paint film thickness of 1.5 mils. If the paint is spray applied with a conventional gun with 20% to 50% transfer efficiency, the paint will actually only cover 60 to 150 square feet. This is because 50% to 80% of the paint that is sprayed does not stick to the part. Due to the velocity of the air and paint mixture leaving the gun tip, much of the paint bounces off of the part. The implications of transfer efficiency are cost and solvent usage. Low transfer efficiency increases the amount spent by a company on paint, increases booth filter cost, and causes the release of more solvents. The theoretical coverage listed on the back of a paint can is the coverage if all of the paint in the can is applied to a part. It is never possible to achieve 100% transfer efficiency.

A separate issue is paint film thickness. If a gallon of paint will theoretically cover 300 square feet at 1.5 mils, it will only cover 150 square feet at 3 mils. If the transfer efficiency of the spraying equipment is 20% to 50%, then the paint applied at 3 mils will only cover 30 to 75 square feet.

Parameters That Affect Transfer Efficiency and Paint Coverage

• Equipment Setup
• Solvent and Solids Content
• Spray Coating Equipment
• Shape of Workpiece
• Changing Conditions
• Operator Training
• Paint Booth

Equipment Setup

A simple description of setting up a conventional spray gun is described below. The different types of spray guns work in a similar manner. Variations of the setup technique will achieve good results for the different gun types. For a conventional spray gun, the required fluid pressure in the paint pot is based on delivering a certain amount of paint through the hoses to the paint gun tip. The best way of setting the pressure is to measure the volume of paint flowing through the gun. Atomizing air is shut off to the gun and the paint pot pressure is set initially to about 6 psi. Trigger the gun into a graduated container and measure the paint volume per minute through the gun. A conventional gun should probably be set at 14 to 16 ounces per minute. Another way is to adjust the pressure visually. The atomizing air is adjusted to zero and the paint gun is triggered. When the stream of paint leaving the gun tip is about 12" long, then the fluid pressure setting is adequate. Higher pressure will result in heavy paint flow and large particle size. In general, the fluid pressure should not be greater than 20 psi. If more pressure is required, a larger paint gun tip is probably needed.

The atomizing pressure is then initially set at 30 psi and a test card is spray painted to inspect for droplet size. If the paint droplets or particles are too large, increase the atomizing pressure by 5 psi and paint another test card. After a certain point, increases in atomizing pressure will not decrease droplet size, but will increase bounce back and paint waste. The goal is to maintain the minimum atomizing pressure and minimum fluid pressure.

Spray Coating Equipment

HVLP spray guns use a larger volume of air at a lower pressure to atomize paint. Fluid pressure is set similarly to conventional guns. Atomizing air pressure is not greatly variable. Adjusting fluid pressure and atomizing pressure, then spray coating a test card will arrive at the correct settings. Paint flow through the gun will be less than a conventional gun. If a conventional gun requires 14 to 16 ounces per minute, an HVLP will require 10 to 12 ounces per minute. HVLP guns will use about 3 times the amount of air as conventional spray guns. A conventional gun will use between 5 and 10 CFM while the same size HVLP will use 15 to 30 CFM.

There are two types of HVLP guns. One type connects directly to compressed air lines for atomization air. Another type uses low pressure air supplied from an electrically powered turbine blower unit. Turbine HVLP guns are considered to have greater transfer efficiency, but are more expensive than standard HVLP guns.

Any HVLP gun coating a flat surface with a moderate viscosity paint will have a minimum transfer efficiency of 65%. A few HVLP guns approach 90%. Some HVLP guns will spray very viscous coatings. The reason for the greater transfer efficiency is low velocity. Low velocity results in less bounce back, less blowby, and reduced paint usage.

Airless guns use no atomizing air. They depend on high fluid pressure from 500 to 3,000 psi to atomize and propel the paint to a surface. Fluid pressure affects air entrapment, fan pattern, and droplet size. The pressure should be adjusted to eliminate air entrapment and to provide the proper fan pattern. Test cards can be used to determine the lowest fluid pressure that provides minimum droplet size. Airless guns are very good at applying heavy coatings, thick film buildup, and high production rates. They are used with good results in conventional situations as well. Transfer efficiency is thought to be up to 30% better than conventional guns, but less than HVLP.

Air Assisted Airless spray guns use some of the good features of both airless and conventional spraying. Typically the fluid pressure is lower than airless. Transfer efficiencies are reported to be slightly better than pure airless guns, but not as high as HVLP.

Electrostatic enhanced application is available in airless, conventional, HVLP, and other spraying methods. Electrostatic application works by providing a negative charge to the atomized paint droplets with the work piece electrically grounded, thus having a less negative charge than the paint. Increased transfer efficiency is achieved, up to 95%. For example, it is possible to spray paint a cyclone fence electrostatically with fair transfer efficiency. Both sides of the fence can be painted from only one side due to the electrostatic attraction. However, recessed areas are difficult to paint electrostatically. Water-based paints are also more problematic when used in electrostatic systems.

Changing Conditions

When a paint shop first opens on a cold winter morning, the paint temperature might be 55EF. As the shop warms, the paint temperature will increase, reducing viscosity. If a paint gun is set up properly in the morning, it may be spraying too much paint in the afternoon. It may be necessary to set up the guns and pot pressure several times per day. The same is true when new or different batches of paint are used. Changes in temperature, humidity, and even barometric pressure can result in subtle or major changes in gun setup requirements.

One way to avoid changing conditions is to use heating bands on paint pots. Not only will the guns remain set up longer, fewer solvents will be needed for thinning. Heat can be used instead of solvent to provide paint thinning with some types of paint.

Solvent and Solids Content

All paints are not created equal. Obviously some cost more per gallon. Some are intended for drywall and others for cars. Assuming that several paints are intended for the same end use, it is important to consider overall theoretical coverage and solvent content in addition to cost per gallon and performance.

Higher solids paints are available that contain fewer solvents by weight and much more resin. For example, the solvent-based paint might weigh 9 pounds per gallon and have a theoretical coverage of 200 square feet. Solvents make up about 84% of the volume in the solvent-based paint, or 6 pounds per gallon. Resins make up only about 16%, or 3 pounds per gallon. In a high-solids paint with 3 pounds of solvent per gallon, solvents make up about 40% by volume. Resins make up about 66% or 9.9 pounds. The high-solids paint has about 3.3 times the amount of resin and coverage compared to solvent-based paint. Theoretical coverage would be 660 square feet in the high-solids paint. If the high-solids paint costs 2 or 3 times more per gallon, it is actually much less expensive on a per square foot basis.

By using a high-solids paint, less paint will have to be sprayed. For example, coating a 1,000 square foot surface using a conventional spray gun with a 50% transfer efficiency, a total of 10 gallons of solvent-based paint or 3 gallons of high-solids paints will be needed to coat the surface. This is shown in the table below. Note that overall solvent emissions are reduced from 60 to 9 pounds and overspray is reduced from 5 to 1.5 gallons.

Sometimes problems associated with using high-solids paints can be eliminated by heating the paint. Increased temperature will result in reduced viscosity and will prevent changing shop conditions from affecting the paint.

Some paints are sold as water-based paints. Water-based paints use water in place of some or all of the solvent used for thinning and dispersion. Many water-based paints also offer increased solids content resulting in greatly increased coverage. In addition water-based paints are comparable to solvent-based paints in performance. Some are as good or better than solvent-based paints in adhesion, scratch resistance, abrasion, salt spray resistance, and UV resistance.

Shape of Workpiece

The greatest transfer efficiency will be achieved by spray painting a flat surface. When theoretical transfer efficiencies are quoted in spray gun catalogs, those are based on painting a flat, wide surface. In real life, the workpiece is seldom flat, has deep holes, and is sometimes inaccessible. Painting a flat wall is easy. Replacing that wall with a cyclone fence is another story. Spray painting a hole has a transfer efficiency of zero. In less extreme conditions, a surface may have deep recesses or other three dimensional shapes. Transfer efficiency is compromised because the spray gun is never at the correct distance from the surface or may be held at an angle to reach a surface. When the gun is too far away, paint can dry before reaching the surface or it may never reach the surface, resulting in overspray. When too close, the paint is applied too heavily and there is increased bounce back. Both decrease coverage.

Operator Training

Spray painting personnel have a tremendous effect on transfer efficiency and coverage. If a painter does everything correctly, the transfer efficiency will be no better than that quoted in the sales catalog. There are some common problems that can be partially solved by training. But don't blame all the problems on the operator. Often painters have not been properly trained or do not have the correct tools such as measuring equipment or high efficiency spray equipment.

• Clearing the Gun -- Triggering the paint gun when the gun is pointed at the floor or ceiling or anywhere but the part being painted wastes paint.

• Moving the Gun in an Arc -- Moving the paint gun in an arc so that ends of the stroke are too far away and too close in the middle. Paint guns should be moved parallel to the surface.

• Too Much or Too Little Overlap -- A 50% overlap pattern is usually recommended to avoid heavy and light areas.

• Thick Coating -- The paint manufacturer may recommend 1.5 mil thickness. Operators may be applying 3 mils. The result is twice as much paint being used.

• Using Incorrect Fan Pattern -- A wide fan pattern is great for wide open spaces. It is not so good for painting narrow edges, because a narrow edge will occupy only a small portion of the fan. The rest of the paint is overspray.

• Cranking up the Pressure -- Increasing fluid and atomizing pressure well above the recommended setting is common. The result is usually more paint being used.

• Holding the Gun at an Angle -- Spray guns should be pointed perpendicular to a surface. Holding it at an angle results in some or all of the spray pattern being too far away from the surface.

Paint Booth

Why is a paint booth important? It is important from a worker exposure, quality, and paint usage perspective. A paint booth is intended to collect overspray paint and to remove solvent fumes from the work area. If painting is done in an area with no ventilation, fumes will build, resulting in a fire and health hazard. Also, oversprayed paint will fall back onto newly painted surfaces causing quality problems. Paint booths also eliminate drafts and odd currents of air that could otherwise carry paint away from the part and onto neighboring cars and buildings.

It is important to provide makeup air for the booth. If a booth exhausts 3,000 CFM, then an equal amount of air must be brought into the building to avoid negative pressure situations. Insufficient makeup air will result in reduced flow through the booth eliminating booth benefits.

Conclusion

There is no one overall solution that will result in minimum paint usage. Efficient application equipment, equipment setup, using high-solids and water-based paints, and employee training will help. It is also important to consider the performance of the paint, and to educate purchasing personnel that low paint cost per gallon or the price of a paint gun may only be part of the equation. Sources of help and decision making on paint, equipment, employee training, and environmental regulations are listed below.

Sources of Technical Assistance

Georgia Pollution Prevention Assistance Division (P²AD) provides confidential, nonregulatory technical assistance to improve manufacturing efficiency and cost reduction at no fee to the customer. P²AD can also put you in contact with Georgia Tech and University of Georgia technical assistance groups that are located near your facility. Phone (404) 651-5120 Fax (404) 651-5130 e-mail p2ad@ix.netcom.com

Georgia Tech Economic Development Institute (EDI) offers assistance with energy and cost reduction in manufacturing situations. EDI has field offices throughout the state. Phone (404) 894-5240

Georgia Tech Research Institute (GTRI) provides assistance with environmental, health, and safety issues and manufacturing technology. Phone (404) 894-3806

Paint and equipment vendors and trade associations have expertise in all aspects of the metal finishing industry. Vendors will often provide technical advice and training for their customers. Some conduct training courses on various aspects of painting for a fee.

Sources of information used to complete this article:

ENVIRO$EN$E Internet website. http://es.inel.org

Coatings Alternative Guide (CAGE) Internet website. http://cage.rti.org

"The ABC's of Spray Finishing " DeVilbiss

"Nordson Miscellaneous Technical Information" Nordson

"Sherwin Williams Chemical Coatings Finishing Systems Guide" Sherwin Williams

Thanks to Bill Allen and Ron Mathews for help in preparing this article.

Bill Allen, Jr.

Southern Fluid Systems

1785-B MacArthur Blvd. NW

Atlanta, GA 30318

(404) 352-2688

 

Ron Mathews

Sherwin Williams

765 North Ave. NE

Atlanta, GA 30306

(404) 873-6723