APPLICATION

7.1 Introduction. This section provides general information on paint application and on activities associated with application such as paint storage and mixing. Application procedures discussed include brushing, rolling, and spraying (conventional air, airless, air-assisted airless, high-volume low-pressure, electrostatic, plural component, thermal, and powder).

7.2 Paint Storage Prior to Application. The installation industrial hygienist should be consulted about local regulations for paint storage, since storage of paint may be subject to hazardous product regulations. To prevent premature failure of paint material and to minimize fire hazard, paints must be stored in warm, dry, well ventilated areas. They should not be stored outdoors, exposed to the weather. The storage room or building should be isolated from other work areas. The best temperature range for storage is 50 to 85 degrees F. High temperatures may cause loss of organic solvent or premature spoilage of water – based paints. Low temperature storage causes solvent-borne coatings to increase in viscosity, and freezing can damage latex paints and may cause containers to bulge or burst. (When paint is cold, a 24-hour conditioning at higher temperatures is recommended prior to use.) Poor ventilation of the storage area may cause excessive accumulation of toxic and/or combustible vapors. Excessive dampness in the storage area can cause labels to deteriorate and cans to corrode. Can labels should be kept intact before use and free of paint after opening so that the contents can readily be identified.

The paint should never be allowed to exceed its shelf life (normally 1 year from manufacture) before use. The stock should be arranged, so that the first paint received is the first paint used. Paint that has been stored for a long period of time should be checked for quality and dry time before use. Quality inspection procedures are described in par. 9.5.5.

7.3 Preparing Paint for Application

7.3.1 Mixing. During storage, heavy pigments tend to settle

to the bottom of a paint can. Prior to application, the paint must be thoroughly mixed to obtain a uniform composition.

Pigment lumps or caked pigment must be broken up and completely redispersed in the vehicle. Incomplete mixing results in a change of the formulation that may cause incomplete curing and inferior film properties. However, caution must be used not to

overmix waterborne paints since excessive foam can be created. Constant mixing may be required during application for paints with heavy pigments, such as inorganic zincs.

Mixing can be done either manually or mechanically.

Two types of mechanical mixers are commonly used: ones which

vibrate and ones which stir with a propeller. Since manual mixing is usually less efficient than mechanical mixing, paints should only be manually mixed when little mixing is needed because there is limited pigment settling or when mechanical mixing is not possible. Vibrator-type mixers should not be used with partly full cans of paint. This can cause air to become entrained in the paint which, if applied, may lead to pinholes in the dry film.

When pigments form a rather hard layer on the bottom of the can, the upper portion of the settled paint can be poured into a clean container (Figure 7), so that the settled pigment can more easily be broken up and redispersed to form a smooth uniform thin paste. When mixing manually, lumps may be broken up by pressing them against the wall of the can. It is essential that settled pigments be lifted from the bottom of the can and redispersed into the liquid. Once the material is uniform, the thin upper portion of the container is slowly poured into the uniform paste while the paint is stirred. Stirring is continued until the entire contents is uniform in appearance. No more paint should be mixed than can be applied in the same day. Paint should not be allowed to remain in open containers overnight.

7.3.1.1 Mixing Two-Component Coatings. Epoxies and polyurethanes are commonly used two-component coatings. The base component, A, contains the pigment, if any. The B component contains the curing agent. The two components must be mixed in the ratio specified by the coating manufacturer on the technical data sheet, unless the coating is being applied using a plural component gun (refer to par. 7.5.4.5). Usually the materials are supplied so that the contents of one can of component A is mixed with the contents of one can of component B. Failure to mix the components in the proper ratio will likely result in poor film formation. Binder molecules are cross-linked in a chemical reaction upon mixing of the two components. Unless the two components are mixed together, there will be no chemical reaction and no curing of the paint.

a) Mixing. Two-component coatings are preferably mixed with a mechanical stirrer as follows:

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Figure 7

Illustration of Mixing and "Boxing" One-Component Paint: A –

Pouring Off Pigment-Poor Vehicle, B and C – Mixing Pigment to Form Smooth Paste, D – Pouring in Vehicle and Mixing,

E – Boxing Paint

 

(1) The base component is mixed to disperse settled pigment. If necessary, some of the thin, upper portion may be poured off before stirring to make it easier to disperse the pigment. When the upper portion is poured off, it must be mixed back with the bottom portion before the two components are mixed together.

(2) While continuing to stir, the two components are slowly mixed together. No more than a few gallons should be mixed at a time, or no more than that specified by the coating manufacturer, since heat is usually generated upon mixing because of the chemical cross-linking reaction. Excessive heat may lead to premature curing of the coating, reducing the pot life.

(3) The two combined parts are agitated until they are of smooth consistency and of uniform color. (Often the color of the two components is different.)

b) Induction. Some two-component paints must stand for approximately 30 minutes after mixing before application.

This time is called the induction time. During induction, the chemical reaction proceeds to such an extent that the paint can be successfully applied. However, some formulations of two – component paints do not require any induction time and can be applied immediately after mixing the two components. Material specifications and manufacturer’s recommendations must be followed carefully. Induction time will depend on temperature of the paint.

c) Pot Life. Pot life is the time interval after mixing in which a two-component paint can be satisfactorily applied. Paints low in VOC content often have a reduced pot life. The chemical reaction that occurs when two component paints are mixed accelerates with increasing temperature. Thus, a paint’s pot life decreases as the temperature increases. Above 90 degrees F, the pot life can be very short. (Curing time of the applied coating is also faster at higher temperatures.) Pot life is also affected by the size of the batch mixed, because the chemical reaction produces heat. The larger the batch, the more the heat produced and the faster the curing reaction proceeds. Thus, the shorter the pot life. Paint must be applied within the pot life. The coating manufacturer’s recommendations must be followed carefully. Mixed two-component paint remaining at the end of a shift cannot be reused and must be discarded. Lines, spray pots, and spray guns must be cleaned during the pot life of the paint.

7.3.2 Thinning. Usually thinning to change the viscosity of liquid paint should not be necessary. A manufacturer formulates paint to have the proper viscosity for application. If thinning is necessary, it must be done using a thinner recommended by the coating manufacturer. Also, the amount used should not exceed that recommended by the coating manufacturer. Prior to adding the thinner, the temperature of the coating and the thinner should be about the same. The thinner must be thoroughly mixed into the paint to form a homogeneous material. Some "false­bodied" or "thixotropic" paints are formulated to reach the proper application viscosity after stirring or during brush or roller application. Undisturbed in the can, they appear gel like, but upon stirring or under the high shear of brush or roller application, these materials flow readily to form smooth films. Upon standing, the coating in the can will again become gel-like. Because of this property, thixotropic coatings may require constant agitation during spray application.

7.3.3 Tinting. Tinting should be avoided as a general practice. If materials are tinted, the appropriate tint base (e. g., light and deep tones) must be used. Addition of excessive tinting material may cause a mottled appearance or degrade the film properties (e. g., adhesion). Also, tinting should only be done with colorants (tints) known to be compatible with the base paint. No more than 4 ounces of tint should be added per gallon of paint.

7.3.4 Straining. Usually, paint in freshly opened containers should not require straining. However, mixed paint having large particles or lumps must be strained to prevent the film from having an unacceptable appearance or clogging spray equipment. Straining is especially important for inorganic zinc coatings. Straining is done after mixing, thinning, and tinting is completed by putting the paint through a fine sieve (80 mesh) or a commercial paint strainer.

7.4 Weather Conditions Affecting Application of Paints.

Paint application is a critical part of a complete paint system. Many of the newer paints are more sensitive to poor application procedures and environmental conditions than oil paints. Four main weather conditions must be taken into account before applying coatings: temperature, humidity, wind, and rain or

moisture. The paint manufacturer’s technical data sheets should be consulted to determine the limits for these conditions as well as other constraints on application of the paint. Applying paints outside the limits is likely to lead to premature coating failure.

7.4.1 Temperature. Most paints should be applied when the ambient and surface temperature is between 45 degrees F and 90 degrees F. Lacquer coatings such as vinyls and chlorinated rubbers, can be applied at temperatures as low as 35 degrees F. There are other special coatings that can be applied at temperatures below 32 degrees F but only in strict compliance with manufacturer’s instructions. Application of paints in hot weather may also cause unacceptable films. For example, vinyls may have excessive dry spray and latex paints may dry before proper coalescence, resulting in mud-cracking. In all cases painting must be done within the manufacturer’s acceptable range. Also, the temperature of the paint material should be at least as high as the surface being painted. Paint should not be applied when the temperature is expected to drop below 40 degrees F before the paint has dried (except when allowed in the manufacturer’s instructions).

7.4.2 Humidity. Ensuring the proper relative humidity during application and cure can be essential for good film performance. However, different types of coatings require different relative humidities. The coating manufacturer’s technical data sheet should be consulted. Some coatings cure by reacting with moisture from the air (e. g., moisture-curing polyurethanes, silicones, and inorganic zincs). These coatings require a minimum humidity to cure. However, too high a humidity may cause moisture-curing coatings to cure too quickly resulting in a poorer film. In addition, too high a humidity may cause blushing (whitish cast on surface of dry film) of some solvent-borne coatings. Blushing is caused when the surface of a coating film is cooled by evaporation of a solvent to such an extent that water condenses on the still wet film. Excessive humidity may also cause poor coalescence of latex coatings since the coalescing agent may evaporate before enough water evaporates to cause coalescence of the film.

7.4.3 Wind. Wind can cause a number of problems during spray application. These include uncontrollable and undesirable overspray and dry spray caused by too fast evaporation of the solvents. The wind velocity at which these undesirable effects occur depends upon the material being applied and the application parameters. Wind can also blow dust and dirt onto a wet surface which could lead to future paint breakdown.

7.4.4 Moisture. Paint should not be applied in rain, wind, snow, fog, or mist, or when the surface temperature is less than 5 degrees F above the dew point. Water on the surface being painted will prevent good adhesion.

7.5 Methods of Application. The most common methods of

application are brush, roller, and spray. They are discussed in detail below. Paint mitts are recommended only for hard to reach or odd-shaped objects such as pipes and railings when spraying is not feasible. This is because it is not possible to obtain a uniform film that is free of thin spots with mitt application. Foam applicators are useful for touch-up or trim work. Dip and flow coat methods are beyond the scope of this handbook. Of the three primary methods, brushing is the slowest, rolling is faster, and spraying is usually the fastest of all. A comparison of approximate rates of application by one painter of the same paint to flat areas is listed in Table 10.

Table 10

Approximate Rates of Paint Application (From SSPC Good Painting Practice)

Method

Square Feet Applied in 8 Hour

Day

Brush

800 – 1400

Roller

2000 – 4000

Air Spray

4000 – 8000

Airless Spray

8000 – 12,000

7.5.1 Selection of Application Method. The choice of an application method depends on the type of coating, the type of surface, environmental factors, and cleanup. Alkyd coatings can easily be applied by brush, but fast drying coatings, such as vinyls, are difficult to apply by brush or roller. Brushing is the preferred method for small areas and uneven or porous surfaces, while rolling is practical on large flat areas. Also, brushing of primers over rusted steel and dusty concrete is preferred over spraying. (Note that applying paint over these substrates should be avoided, if possible.) Spraying is usually preferred on large areas and is not limited to flat surfaces. Spraying may not be feasible is some locations and in some environments because of the accumulation of toxic and flammable fumes or overspray.

7.5.2 Brush Application. Brushing is an effective method of paint application for small areas, edges, corners, and for applying primers. Brush application of primers works the paint into pores and surface irregularities, providing good penetration and coverage. Because brushing is slow, usually it is used only for small areas or where overspray may be a serious problem.

Brush application of paint may leave brush marks with paints that do not level well, thus creating areas of low film thickness.

Even a second coat of paint may leave the total coating system with thin and uneven areas that may lead to premature failure.

Brushes are made with either natural or synthetic bristles. A drawing of a typical paint brush is shown in Figure 8. Chinese hog bristles represent the finest of the natural bristles because of their durability and resiliency. Hog bristles are also naturally "flagged" or split at the ends. This permits more paint to be carried on the brush and leaves finer brush marks on the applied coating. Horsehair bristles are used in cheaper brushes but are an unsatisfactory substitute for hog hair. Nylon and polyester are used in synthetic bristles or filaments. The ends are flagged by splitting the filament tips. Synthetic bristles absorb less water than natural bristles and are preferred for applying latex paints. However, synthetic bristles may be softened by strong solvents in some paints.

Thus, natural bristles are preferred for application of paints with strong solvents.

Brushes are available in many types, sizes, and qualities to meet the needs for different substrates. These types include wall, sash and trim (may be chisel or slash­shaped), and enamel (bristles are shorter). It is important to use high quality brushes and keep them clean. Brushes with horsehair or with filaments that are not flagged should be avoided. The brush should be tapered from side to center (see Figure 8).

7.5.2.1 Procedure for Brush Application

a) Shake loose any unattached brush bristles by spinning the brush between the palms of the hand and remove the loose bristles.

b) Dip the brush to cover one-half of the bristle length with paint. Remove excess paint on the brush by gently tapping it against the side of the can.

c) Hold the brush at an angle of about 75 degrees to the surface. Make several light strokes to transfer the paint to the surface. Spread the paint evenly and uniformly. Do not press down hard but use a light touch to minimize brush marks.

If there is time before the paint sets up, cross-brush lightly to eliminate excessive brush marks.

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Figure 8

Illustration of Parts of Paint Brush

d) Confine painting to one area so that a "wet edge" is always maintained. Apply paint to a surface adjacent to the freshly painted surface sweeping the brush into the wet edge of the painted surface. This helps to eliminate lap marks and provides a more even coating film.

7.5.3 Roller Application. Roller application is an efficient

method for flat areas where the stippled appearance of the dry film is acceptable. However, paint penetration and wetting of difficult surfaces is better accomplished by brush than roller application. Thus, brush application of primers is preferred over roller application.

A paint roller consists of a cylindrical sleeve or cover which slips onto a rotatable cage to which a handle is attached. The covers vary in length from 1 to 18 inches and the diameter from 1.5 to 2.25 inches. A 9-inch length, 1.5-inch diameter roller, is common. The covers are usually made of lamb’s wool, mohair, or synthetic fibers. The nap (length of fiber) can vary from 0.25 to 1.25 inches. Longer fibers hold more paint but do not give as smooth a finish. Thus, they are used on rougher surfaces and chain link fence, while the shorter fibers are used on smooth surfaces. Use of extension handles makes the application of paint to higher surfaces easier.

However, use of a long extension handle usually results in a less uniform film. Use a natural fiber roller (for example, wool – mohair) for solvent base paints and a synthetic fiber roller for latex paints.

7.5.3.1 Procedures for Roller Application. Rollers are used with a tray which holds the coating or a grid placed in a 5- gallon can (Figure 9). Application procedure is described below.

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Figure 9

Equipment Used in Applying Paint by Roller

a) If a tray is being used, fill it half full with premixed paint. If a grid or screen is being used, place it at an angle in the can containing premixed paint.

b) Immerse the roller completely in the paint and remove the excess by moving the roller back on the tray or grid. Skidding or tracking may occur if the roller is loaded with too much paint.

c) Apply the paint to the surface by placing the roller against the surface forming a "V" or "W" of a size that will define the boundaries of the area that can be covered with the paint on a loaded roller. Then roll out the paint to fill in the square area. Roll with a light touch and medium speed.

Avoid letting the roller spin at the end of a stroke. Always work from a dry adjacent surface to a wet surface. The wet edge should be prevented from drying to minimize lap marks.

d) Use a brush or foam applicator to apply paint in corners, edges, and moldings before rolling paint on the adjacent areas.

7.5.4 Spray Application. Spray application is the fastest

technique for applying paint to large areas. Spray application also results in a smoother, more uniform surface than brushing or rolling. There are several types of equipment: conventional

air, airless, air-assisted airless, high-volume, low-pressure (HVLP), electrostatic, multi-component, thermal, and powder. Conventional air and airless were most commonly used. However, with changing VOC requirements the other methods are being used more. Air or air-assisted methods of spraying, including HVLP, rely on air for paint atomization. Jets of compressed air are introduced into the stream of paint at the nozzle. The air jets break the paint stream into tiny particles that are carried to the surface on a current of air. The delivery of the paint to the nozzle may be assisted using hydraulic pressure. In airless spray, paint is forced through a very small nozzle opening at very high pressure to break the exiting paint into tiny droplets. A general comparison of properties of conventional air and airless spray are given in Table 11. Note that specific application rates, the amount of overspray, and other properties depend to a great extent upon the type of paint, and may vary from those listed in the table. Air methods other than conventional have been developed to overcome some of the environmental and other concerns of air and airless spray. These differences are discussed separately for each method below.

7.5.4.1 Conventional or Air Spray Equipment. The conventional method of spray application is based on air atomization of the paint. The basic equipment (air compressor, paint tank, hoses for air and paint, spray gun) is shown in Figure 10. The coating material is placed in a closed tank (sometimes called a pot) connected to the nozzle by a hose and put under regulated pressure using air from the compressor. A hose from the air compressor to the nozzle supplies the air required for atomization of the paint. The tank may be equipped with an agitator for continuously mixing paints with heavy pigments. The air compressor must have sufficient capacity to maintain adequate and constant air pressure and airflow for paint atomization at the nozzle, for paint flow from the tank to the nozzle, for powering the agitator and other job-site requirements. A constant flow of air from the compressor is required for proper painting. Loss of pressure at the nozzle can cause pulsating delivery of the paint as opposed to the desired constant flow. (Data sheets from paint manufacturers give recommended air pressures for spraying.)

a) Air Hose. The air hose connecting the compressor to the tank must be of sufficient diameter to maintain adequate air pressure. Required diameter of the fluid hose connecting the gun and tank depends on volume and pressure of paint required at the gun. The hose should be kept as short as possible, especially when spraying coatings with heavy pigments, to avoid settling of pigments within the supply hose. Also, the fluid hose must be resistant to paints and solvents that flow through it. As with blasting equipment, the air supply must be free of moisture, oil, and other impurities. Oil and water should be removed by separator or extractor attachments to the compressor.

Table 11

Comparison of Conventional Air and Airless Spray

Property

Conventional Air

Airless

Coverage, sq ft/day

4-8000

6-10,000

Overspray

Considerable

Some

Transfer efficiency

Poor (about 30

Fair (35-50)

"Bounce back"

percent)

Minor

Hoses

Significant

1 (fluid)

Penetration of corners,

2 (air and fluid)

crevices and cracks

Moderate

Film build per coat

Fair

Good

Versatility

Fair

Fair

Paint clogging problems

Good

More

Operator control

Few

Poor

Safety during painting

Good

Poor

Safety during cleanup

Fair

Fair

Poor

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Figure 10

Schematic Drawing Illustrating Basic Parts of Conventional Air Spray Application Equipment

b) Gun or Nozzle. The gun or nozzle is a relatively complex device (Figure 11). It consists basically of 10 parts:

(1) Air nozzle or cap that directs the compressed air into the stream of paint to atomize it and carry it to the surface.

(2) Fluid nozzle that regulates the amount of paint released and directs it into the stream of compressed air.

(3) Fluid needle that controls the flow of fluid through the nozzle.

(4) Trigger that operates the air valve and fluid

needle.

(5) Fluid adjustment screw that controls the fluid needle and adjusts the volume of paint that reaches the fluid tip.

(6) Air valve that controls the rate of airflow through the gun.

(7) Side port control that regulates the supply of air to the air nozzle and determines the size and shape of the spray pattern.

(8) Gun body and handle designed for easy

handling.

(9) Air inlet from the air hose.

(10) Fluid inlet from fluid hose.

c) Air Nozzle. Two general types of air nozzles are

available: external atomization and internal atomization. In

both types, outer jets of air atomize the wet paint (see Figure 12). In the external type, paint is atomized outside the nozzle, while in the internal type paint is atomized just inside the nozzle opening. The type selected depends on the type of paint to be sprayed and the volume of air available. The external type is the more widely used. It can be used with paints and most production work. The spray pattern can be adjusted. A fine mist can be obtained which can result in a smooth even finish. Nozzle wear and buildup of dry material are not major problems. The internal-mix air nozzle requires a smaller volume of air and produces less overspray and rebound than the external type. The size and shape of the spray pattern of the internal-mix nozzle cannot be adjusted. Catalyzed and fast drying paints tend to clog the openings of internal-air nozzles. These coatings should be sprayed with an external-mix nozzle.

d) Setting-Up, Adjusting Equipment, and Shutting-Down Procedures. Both the pressure on the paint and the air pressure at the gun must be properly regulated to obtain the optimum in film performance. A properly adjusted nozzle will produce a fan that is about 8 inches wide, 10 inches from the gun. The shape of the spray pattern produced may vary from round to oval. The pattern must have well defined edges with no dry spray at the ends or heavy film buildup in the middle (Figure 13). Coating manufacturers provide guidance on appropriate equipment and pressures for application of their coatings. Additional job-site adjustments may be necessary. The aim is to obtain a wet looking film that is properly atomized with as little overspray as possible. To minimize bounce back and dry spray, the atomizing air pressure should be kept as low as possible. Common spray pattern problems and their cause and remedy are listed in Table 12. The gun should be taken apart and cleaned at the end of each day and the air cap and fluid tip should be cleaned with solvent. Pivot points and packing should be lubricated with lightweight oil. Leaving a gun in a bucket of solvent overnight will likely cause the gun to become plugged and lead to premature failure of the gun. The shutting-down procedure is detailed in the instructions supplied by the manufacturer of the spray equipment and these instructions should be followed. Other worker safety issues are discussed in the section on safety.

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Figure 11

Drawing of Air-Spray Gun

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Figure 12

Cross-Sectional Drawing of Nozzle of Air-Spray Gun

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Illustration of Proper Spray Patterns (Note that the patterns are uniform throughout.)

 

Table 12

Common Conventional Air-Spray Problems and Their Causes

and Remedies

 

Remedy

 

Description Cause

 

Thick center; thin ends; pinholes

 

Atomizing air Increase air pressure;

pressure too low; decrease fluid

too much fluid pressure or use

to gun smaller nozzle

 

Hourglass shape; dry spray on ends

 

Fluid pressure low; air pressure too high; too wide a spray pattern

 

Increase fluid pressure; reduce air pressure; adjust pattern control; reduce paint viscosity

 

Teardrop shape; thicker at bottom

 

Remove and clean air nozzle; replace any bent parts or tighten air nozzle

 

Problem with gun – nick in needle seat; partially clogged orifice or slightly bent needle or loose nozzle

 

Dried paint has clogged one of the side port holes of the air nozzle

 

Dissolve dried paint with thinner; do not probe into nozzle with metal devices

 

"Boomerang"

 

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7.5.4.2 Airless Spray

a) Equipment. Airless spray relies on hydraulic pressure alone. Atomization of paint is accomplished by forcing the material through a specially shaped orifice at pressures between 1000 and 3000 psi. Because of the high pressures, extreme care must be taken to prevent worker injury. The spray manufacturer’s instructions must be followed carefully. The basic parts of airless spray equipment are a high-pressure paint pump, a fluid hose, and an airless spray gun. The high-pressure pump must deliver sufficient pressure and material flow to produce a continuous spray of paint. The fluid hose must be able to withstand the very high pressures required to deliver the paint to the gun and atomize it. A filter screens out particles that might clog the tip. Since atomization is controlled by the size and shape of the orifice of the tip, a different tip is used to obtain different patterns and atomization rates. The tip angle controls the fan width. Tips having the same orifice size but different angles deliver the same amount of paint, but the area covered with one pass is different. Viscous materials require a larger tip than less viscous materials. Coating manufacturers recommend tip sizes on their data sheets. The larger the orifice, the greater the production rate. But, if too large an orifice is used for a thin coating, the rate of delivery may be such that the operator cannot keep up with the flow. This will result in sagging and running of wet paint. Airless spray is available with heaters to reduce paint viscosity, permitting spraying of coatings having higher ambient viscosities at a faster production rate.

b) Setting-Up, Adjusting Equipment, and Shutting-Down

Procedures. The manufacturer’s instructions should be followed for setting up the spray equipment. To minimize tip clogging problems, airless spray equipment must be scrupulously clean before setting-up for a spray application and the coating must be free of lumps. The manufacturer’s recommendations should be followed rigorously for the setting-up, using, and shutting-down procedures. Since the pressures used are high, two safety features are required for guns: a tip guard and a trigger lock.

The tip guard prevents the operator from placing a finger close to the tip and injecting paint into the skin. The trigger lock prevents the trigger from accidentally being depressed. Other safety measures include never pointing the gun at any part of the body; not making adjustments without first shutting off the pump and releasing the pressure; making sure the fluid hose is in good condition, free of kinks, and bent into a tight radius; and using only high-pressure hose fittings. Also, never clean systems containing aluminum with chlorinated solvents. Explosions may occur. Causes of and remedies for faulty patterns are described
in Table 13. Additional problems that may occur with airless spraying may be associated with excessive pressure, undersized equipment, and too long or too small paint hoses. Undersized spray equipment, including hoses, may result in lower production rates, a pebbly-appearing film caused by poor atomization (nozzle tip too large), and thin films. Air supply hoses that are too long or too small may cause instability of the pump, poor atomization of the paint, or a pulsating spray pattern.

Table 13

Common Airless-Spray Problems and Their Causes and Remedies

Подпись:APPLICATIONПодпись:Подпись: decrease paint viscosity, choose larger tip orifice, or reduce number of guns using one pump

APPLICATION APPLICATION

Description Cause

7.5.4.3 Air-Assisted Airless Spray. Air-assisted airless spray uses air to help atomize paint as compared with only fluid pressure in the airless system. Thus, a lower hydraulic pressure (typically 500 to 1000 psi) can be used. Air pressure is typically 10 to 15 psi. Air-assisted airless spray provides a finer spray than airless spray, and the lower hydraulic pressure provides improved operator control. Consequently, finishes tend to be smoother with fewer runs and sags. Transfer efficiency is about the same as airless spray, but air-assisted airless spray is more expensive to maintain.

7.5.4.4 High-Volume, Low-Pressure Spray. HVLP spray is an air

spray technique that uses low pressure and large volumes of air to atomize the paint. It has much better transfer efficiency that conventional air spray and some systems have been found to meet the 65 percent transfer efficiency requirement of California’s South Coast Air Quality District. Because of the lower air pressures, there is also less bounce back than with conventional systems. Turbine air-supply systems, along with large (1-inch diameter) hoses are commonly supplied with the systems. Since the air supply is not turned off when the trigger is released, air flows continuously through a bleeder valve in the gun. An HVLP gun can be equipped with different fluid and air tips depending upon several variables: the desired spray

pattern (wide fan to narrow jet), viscosity of the finish, and output of the turbine. Although some special training of painters may be required because of differences between conventional air systems and HVLP, such as less recoil, higher delivery volumes and continuous flow of air, an experienced operator has good control.

Conversion kits for air compressor systems are available which allow the use of them with HVLP systems. Spray techniques may be slightly different depending upon the source of pressurized air.

7.5.4.5 Multi-Component Spray. Multi-component (or plural – component) spray equipment combines components of multi-component paints in the nozzle. The equipment is more complicated than other spray equipment, and its use is usually confined to large or specialized coating applications. The components are metered to the gun in the proper relative volumes, mixed and then atomized by one of the previously described techniques. Thus, pot life is not a factor in application of multi-component coatings. However, it is essential that the metering be done in accordance with the coating manufacturer’s instructions. Volume mixing ratios are usually from 1:1 to 1:4. Heating of the components before mixing is also provided with some equipment.

By heating the components, both the viscosity during application

and the cure time can be altered. The equipment is cleaned by purging with solvent. Because of the complicated nature of the equipment, specialized operator training and skilled operators are required. Initial and maintenance costs are also greater than for other spray techniques.

7.5.4.6 Electrostatic Spray. In hand-held electrostatic spray systems, a special protruding part of the gun is given a high, negative voltage which places a negative charge on the spray droplets as they come from the gun. The surface being painted is grounded. This causes the paint droplets to be attracted to the grounded surface to be painted. Because there is an electrical attraction between the paint droplets and the object being painted, a very high percentage of droplets lands on the surface. That is, the transfer efficiency is high and there is minimal overspray. Also, some droplets will be attracted to the edges and the back of the surface, if they are accessible. This is called the wraparound effect. Specially formulated paints are required for electrostatic spraying. Also, painting is restricted to use on conductive substrates, such as steel or galvanized steel. Only one coat of paint may be applied to the base metal by electrostatic spraying since a painted surface is not conductive. Electrostatic spray is an ideal spraying method for piping, fencing, channels, and cables because of the wraparound effect and minimal overspray. However, because of high voltage, special safety requirements must be met, including grounding the power supply and the operator.

7.5.4.7 Powder Spraying. Powder coatings, usually epoxies, are specially prepared polymeric coatings. They are applied to preheated conductive surfaces, such as steel, by special electrostatic spray equipment or in a fluidized bed. Once applied, the coated component is heated to fuse the powder into a continuous coating film. This technique is commonly used in shop applications because heating can be done in an oven, there are no volatile solvents to control and material that did not stick to the surface can be collected and reused. Portable systems are also available and can be used in special situations.

7.5.4.8 Thermal Spraying. Thermal spraying, sometimes called metallizing, is a process in which finely divided metals are deposited in a molten or nearly molten condition to form a coating, usually on steel. Equipment and techniques are available for flame or electric arc spraying of pure zinc, pure aluminum, or an 85 percent zinc, 15 percent aluminum alloy. The coating material is available in the form of a powder or wire, with wire used more frequently. Once the metal becomes molten, it is delivered to the surface with air or gas pressure. It forms a porous coating that protects steel by cathodic protection in a variety of environments. For more severe service such as very acid or alkaline conditions, or fresh or salt water spray, splash, or immersion, the coating may be sealed with a thin conventional organic coating or silicone. A white-metal blasted surface is required. Metal spray coatings are normally very abrasion resistant and provide excellent corrosion control.

Thermal spraying of metals is best accomplished in a shop environment, but can also be done in the field. DOD-STD-2138(SH) describes the wire flame spraying of aluminum using oxygen-fuel gas. SSPC Guide 23, Coating Systems describes thermal spray metallic coating systems.

7.5.4.9 Application Technique. Proper application technique is essential for obtaining quality films. Poor technique can result in variations in paint thickness, holidays (small holes), and other film defects, and wasted time and materials. The same basic techniques described below are used for both conventional and airless spraying:

a) Stroking. With the spray gun at a right angle to the work, the wrist, arm, and shoulder are moved at a constant speed parallel to the surface. Holding the gun at an upward or downward angle to the surface will result in a non-uniform coating thickness and may increase the problem with dry spray or overspray. Also, changing the distance between the gun and the surface, arcing, as illustrated in Figure 14, will result in a non-uniform coating thickness. For large flat surfaces, each stroke should overlap the previous one by 50 percent as shown in Figure 15. This produces a relatively uniform coating thickness. The stroke length should be from 18 to 36 inches, depending upon the sprayer’s arm length and comfort. Surfaces of greater length should be divided into smaller sections of appropriate length (Figure 16). Each section should slightly overlap the previous one along the lines where they are joined.

b) Triggering. The spray gun should be in motion before triggering and continue briefly after releasing at the end of a stroke. This is illustrated in Figure 17. Proper triggering also keeps the fluid nozzle clean, reduces paint loss, prevents heavy buildup of paint at corners and edges, and prevents runs and sags at the start and end of each stroke.

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Figure 14

Illustration of Improper Movement of Spray Gun When Applying Paint

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Figure 15

Illustration of Proper Procedure for Spray Painting

Large Flat Surfaces

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Figure 16

Schematic to Illustrate Proper Painting of Large Vertical

Surfaces

 

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Figure 17

Illustration of Proper "Triggering" of Spray Guns

c) Distance. Distance between the nozzle and the surface being painted depends on atomization pressure and the amount of material delivered. This distance usually varies from 6 to 12 inches for conventional spraying and from 12 to 15 inches for airless spraying. If spray gun is held too close to the surface, heavy paint application and sagging or running may occur. If the gun is held too far away from the surface, a dry spray with a sandy finish may result. Such paint films usually contain holidays (small holes) and provide an unacceptable surface.

d) Corners. Both inside and outside corners require special techniques for uniform film thickness. Each side of an inside corner should be sprayed separately as shown in Figure 18. Too thick a layer of paint can easily be applied to an inside corner. But when too thick a layer is applied, the coating may shrink or pull away from the inside corner causing a void underneath the coating. This will lead to premature failure. An outside corner is first sprayed directly, as shown in Figure 19, and then each side is coated separately. On an outside corner, the coating tends to pull away from the corner. Thus, the coating on the corner tends to be too thin. Outside edges should be ground so that the edge is rounded before painting.

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Figure 18

Proper Spray Painting of Inside Corners

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Figure 19

Proper Spray Painting of Outside Corners

e) Welds. Welds are usually rougher than the adjacent steel and a uniform coating is more difficult to achieve.

Failure often occurs first over welded areas. Thus, after grinding the welds to smooth them, a coat of paint should be brushed over the welds. Then the entire surface can be painted by spray. With this extra coating over the welds, paint often lasts as long over welds as on the adjacent flat areas.

f) Nuts, Bolts, and Rivets. It is a good coating practice to brush-coat these areas before spraying the flat areas. Paint can be worked into crevice and corner areas. Nuts, rivets, and bolts should be sprayed from at least four different angles to prevent thin coatings caused by shadowing effects (Figure 20).

g) Common Errors. Some common errors and the results that are produced in spray painting are summarized in Table 14.

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Figure 20

Schematic Illustrating Importance of Spraying Surfaces With Protruding Parts From All Directions to Avoid "Shadowing Effect"

 

Table 14

Spray Painting Errors

Error

Result

Improper spraying technique (e. g., arcing, tilting gun)

Spray pattern varied from narrow to wide Variation of sheen from overspray Uneven film thickness

Improper fan width

Inadequate or excessive film build on

complex substrate shapes, such as "I" or "H" beams

Spray gun too close to surface

Excessive film build Runs, curtains, sags

Poor paint adhesion from improper curing Wrinkling during and after surface curing Excessive paint used Orange peel pattern or blow holes

Spray gun too far from surface

Film build too thin Non-uniform film thickness Dry spray

Uneven angular sheen from overspray earlier work

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