SPRAY GUN TECHNIQUE - spraytec.net Information/Spraying Techniques.pdf · SPRAY GUN TECHNIQUE PAGE 3 SPRAY GUN TECHNIQUE PAGE 3 with the work surface, causing excessive spray dust

SPRAY GUN TECHNIQUE PAGE 1


SPRAY GUNTECHNIQUE

Steve Mannouch Oct 95 Issue 2



Spray Gun Motion Study

In finishing any component the mostefficient combination of spray gunmovements must be carefully planned inorder to produce the best results. At thesame time the parameters within whichthe gun operates best must also beconsidered. The following notes areintended to highlight the best way to usethe gun. Before commencing spraying it isadvisable that an operator becomesfamiliar with the gun and surface to becoated by spraying some practice panelsto save time and rejects during laterproduction.

The Stroke (Pass)

The spray gun stroke, or pass, is made by

Fig 1a

moving the gun parallel to the work andperpendicular (at right angles) to thesurface. The distance from the gun to thework, for Conventional Air Atomisingguns, should normally be between 6 and8 inches. A simple way to measure this touse your hand. A closed hand (withfingers and thumb together) measuresapproximately 5 inches wide and an openhand (with fingers and thumb apart)measures approximately 8 inches (asshown in figures 1a and 1b).

Spraying should be carried out withstraight uniform strokes moving across

the surface in such a way that the spraypattern overlaps the previous stroke by aminimum of 50%. While this percentage

Fig 1b

may need to be increased for certaincoating types, once established, it mustremain constant for the duration of thework. Failure to do so will result in unevencoating thickness and poor visualappearance. If the gun is held too close tothe work surface, more paint will be

Fig 2.

deposited and the gun will need to bemoved faster to prevent runs and sagsand possible solvent entrapment.Likewise, if the gun is held too far fromthe work, the atomised droplets may drycompletely before coming into contact



with the work surface, causing excessivespray dust and dry spray, (as shown inFigure 2).The relation between the gundistance and the stroke speed is easier tosee when actually spraying. The sprayerwill soon become experienced in movingthe gun target distance closer or furtheraway to permit a comfortable speed,while depositing a full wet coating ofmaterial. Remember at all times that theConventional Air Atomising gun is

Fig 3.

designed to spray at 6 - 8” inches anddeviation from this distance by alteringfluid flows and air pressures is not goodpractice. It could lead to other paint defectproblems.

The gun must not be excessively arced ortilted while spraying, as shown in Figures3 and 4. In certain types of work it isnecessary to tilt the gun but this shouldonly be done when it is not possible touse the correct gun position.

Holding the Gun

The gun must be held at right angles tothe sprayed surface at all times. Figure 4shows the correct position in which thegun should be held, and also the incorrectin dotted lines. It is especially important toremember this when spraying particularly

Fig 4Spray gun should be held perpendicular to thesurface as shown here by solid lines. Tilting the gunup or down gives an uneven spray pattern andmaterial deposition

low or high surfaces where holding thegun at right angles takes more effort onthe part of the sprayer. Failure to do thiswill result in heavy and light bands ofmaterial caused by the deformed spraypattern shape brought about by theincorrect angle of the gun.

Triggering

The gun trigger controls the quantity ofmaterial emitted from the fluid tip, and theoperator needs to learn how to correctlyuse it to achieve the most efficientspraying technique. As the trigger ispulled back the flow of material increases.Continuous adjustment of the fluid flow isnormally necessary when sprayingcomplicated or intricate components. It istherefore important to become familiar tothe trigger ‘feel’ to prevent deposition oftoo much coating on certain areas of thecomponent. Correct use of the trigger willalso reduce the amount of paint wastedand contamination in the spray area.



Spraying a Panel

The basic technique of spraying a flatpanel is shown in Figure 5. The stroke isstarted off the work and the trigger ispulled only when the gun is moving andabout to pass over the edge of the panel.The trigger is released at the other edgeof the panel but the stroke is continued afew inches before reversing direction forthe second pass. The point at which thegun is triggered is one of the mostimportant factors of the spray technique.The gun must follow the precise shape ofthe work, maintaining full coverage overthe wok with minimum waste in overspray.

Fig 5When spraying a panel use alternate right and leftstrokes, triggering the gun at the beginning and endof each stroke. The spray pattern should overlap atleast one-half of the previous stroke for evencoverage without bands.

In order to reduce overspray and wastesome sprayers use an ‘end stoke’technique as shown in Figure 6. The firstsingle vertical stroke at each end of thepanel allows the gun to be triggered offjust before the panel edge. This methodstill assures complete coverage but helpsreduce paint waste which results whenspraying up to and over the vertical edgewith the usual horizontal strokes.

.Long work

Fig 6 End Strokes - Vertical strokes sprayed at theends of a panel help reduce overspray fromhorizontal strokes

A long panel can be sprayed with verticalstrokes, but most sprayers have bettercontrol with the more natural horizontalstroke. The panel should therefore besprayed in several separate sectionstypically 18 to 36 inches long (acomfortable reach of the sprayers gunarm) as shown in Figure 7. This methodwill use the same triggering action as witha smaller panel, but each

section must overlap the previous one byabout four inches. By ‘feathering’ the gun

Fig 7Long Work is sprayed in sections of convenientlength, each section overlapping the previoussection by approx 4 inches



trigger and slightly ‘arcing’ the gun for thisfour inches, heavy material depositionfrom two full coats can be avoided. Donot attempt to spray the complete panellength in one pass - this will easily lead tooverlap and target distance errors.

Horizontal Surfaces

When spraying horizontal surfaces oneoption is for the spray strokes to start atthe near side of the work as shown inFigure 8.

This is essential where rapidly dryingmaterials (e.g. cellulose lacquer) are

Fig 8Start at near edge to avoid depositing dry spray onalready coated work.

used, as drying droplets from thebounce-back landing on wet work can drywith a sandy finish. A very small amountof gun tilt is usually needed for horizontalsurfaces to ‘sweep’ the bounce-backtoward the far edge. The newly depositedwet coat will tend to bury any dry sprayalready deposited on the surface.

The opposite action is also acceptable (iestarting at the far side and ‘sweeping’towardss the sprayers body. However, thesprayer himself will become covered indry spray.

When spraying horizontal surfaces toowide to be coated from a single side it iscommon to start at the near side and

spray towards the middle using the‘sweeping’ technique described above.On reaching the middle the sprayer willmove around to the far side and sprayfrom the middle to the new near edge,again ‘sweeping’ the dryspray, but thistime towards the sprayers body.

This latter technique is typically usedwhen spraying the roof and bonnet ofcars, as it has the advantage of keepingthe ‘wet edge’, allowing wet paintdeposited by spray strokes to merge intothat deposited by the previous strokeacross the complete surface width.However, care must be taken not to allowthe sprayer or the gun hose to touch thewet surface while spraying.

Tilting the Gun

Where a suction feed spray gun is usedon work that requires the gun to be tilted,care must be taken to ensure that the tiltis not excessive as the paint may clog theair vent in the cup lid and interfere withmaterial flow. As an alternative the air capcan be rotated to keep the gun and cupvertical while the spray fan is angled.

If spraying at an upward angle for anylength of time one option is to rotate thecomplete cup assembly through 180degrees so that the air vent lies in front ofthe fluid inlet, and the fluid tube, withinthe cup, curves to where the material willlie when the gun is tilted. An even betteridea is to use a pressure feed system.



Outside Edges and Corners

Fig 9Edges and Corners should be done first, sprayingdirectly at the corner to catch both surfaces withone stroke of the gun. The centre is sprayed like aplain panel.

When a panel is to be sprayed on theedges as well as on its faces a modifiedEnd Stroke technique is used as shown inFigure 9. One stroke along each edgecoats part of the faces as well as theedges at the same time.

Fig 10Spraying the outside corner of a box. Adjoiningsurfaces are coated using a single ‘End Stroke’ ofthe gun.

An outside corner of a box or cabinet istreated in a similar manner, as shown inFigure 10.

Inside Corners

If an inside corner is sprayed square-on,as shown in Figure 11, the coating will

Fig 11Spraying directly into the corner gives an unevencoating but is satisfactory for most work.



Fig 12Spraying each side of the corner separately givesan even coating. Use a standard vertical spraypattern

not be truly uniform but it will be appliedquickly and will be satisfactory for mostwork. To overcome ‘Air Cage’ producedby air atomising guns it is normal for thegun, and therefore the spray fan, to beinclined at a slight angle to increasepenetration and coverage.

When a more even coating is necessary,each face of the corner is sprayedseparately as shown in Figure 12. Aftermaking the first vertical stroke next to thecorner, horizontal strokes can then beused to cover the area adjacent in orderto avoid overspraying or double coatingthe adjoining surface.

Slender Work

When spraying slender work the spraypattern must be adjusted to suit thecomponents being sprayed. A widehorizontal spray pattern must not be usedon a narrow work. A smaller horizontalpattern or (sometimes) a large vertical fanspray gives complete coverage withoutexcessive overspray. However the use ofa spray pattern that is too small for thecomponent must also be avoided orbanding and uneven material depositioncould occur.

Jetting

Occasionally the smallest size fan orround spray is used to concentrate paintand air flows in order to ‘throw’ or ‘jet’ thematerial over increased target distances.Although increased orangepeel will normally result this method isuseful for coating awkward areas that areinaccessible or beyond normal sprayingreach.

Fig 13Avoid excessive overspray - adjust the spraypattern to fit the job. The centre illustration showsthe best method for most work.

Open Work

Iron grilles, fences and similar workshould be sprayed with the intent to getthe material on the largest area with eachstroke of the gun.

Fig 14Open work is usually sprayed at an angle to coveras much of the work as possible with each spraygun stroke.

A picket fence can be sprayed with asingle up-down angle stroke on each



side. A wire fence or other thin, intricate,work should also be sprayed at an acuteangle as shown in Figure 14. It is helpfulto use a shield behind the wire as paintdeflected by bounce-back will help to coatthe back of the work.

The idea of using paint deflected bybounce-back to coat difficult areas ofcomponents is often used duringproduction spraying. However, it must berealised that the deflected paint dropletswill have evaporated much of their solventcontent before finally landing andtherefore a orange peel finish is oftenobtained.

Round work

A flat round disc is sprayed like any otherplane surface, spraying the edge asshown in Figure 6, and then spraying thecentre.

Small, cylindrical shapes such as tableleg turnings are ideally sprayed with around spray pattern, vertical stroke, usingthree or four lapping strokes to obtain fullcoverage, as shown in Figure 15. Avertical fan pattern can be used inconjunction with the vertical stroke, butthe movement of the gun must beincreased to prevent runs or sags.

Large diameter cylinders are usuallyfinished in the same manner as a flatpanel except that the strokes are shorter,as shown in Figure 16. A small diametercylinder is more efficiently sprayed withlengthways strokes as shown in Figure16.

Fig 15Use a round spray or vertical fan pattern for tableleg turnings or similar small round surfaces

Continuous Spray Motion

An important element that has beenfound to greatly increase the efficiency ofspraying operations is the principle ofkeeping continuity of motion, from thetime the gun is triggered when starting tospray, until the gun is triggered off andthe component is completely covered.

Fig 16Round work should be treated to curve the strokesto conform with the surface

This does not necessarily mean that theentire article should be sprayed withoutshutting off the gun, however whenpossible this should be done.

Mounting certain types of work on aturntable which can be revolved easilycan allow an object to be sprayed withone or two simple motions withoutshutting off the spray gun more than onceor twice.



Fig 17Finishing a wheel hub cap in one continuous strokeusing a rotating work holder.

Such continuous, or nearly continuous,operation of the gun will assure maximumcoverage per unit of time and so willproduce a maximum output from thespraying operation.

In such work, it is usually necessary toprovide a turntable with mounting jig tohold the object securely during paintingso that it may be rapidly positioned forcomplete access to all portions. Then, byworking out sequence of motions for thespray gun, complete coverage of thecomponent can be attained with a fewrapid strokes and simultaneous, oralternate, movements of the fixtureholding the work.

Fig 18Showing the path of a spray gun for most efficientspraying with the minimum of strokes

Figures 17 and 18 show some typicalexamples of the recommended practice inspray gun motion and show how acomponent can, in many cases, beefficiently covered by one or morecontinuous strokes of the gun.

Where the object to be sprayed is of sucha shape that it can be rotated while it issprayed, the gun can be held open frombeginning to end of the stroke. Figure 17shows a method used in spraying metalhub caps of wheels. The work is held androtated by the turntable. A 5 inch spraypattern is used. As the hub rotates theoperator simply pulls the trigger back andmoves the gun in a 2 inch arc, finishingthe cap in one continuous operation.

It is not necessary to zigzag this stroke ormove from top to bottom and back again.In the example shown the top half of thehub cap is finished on the first revolution,and the second half by slight movementof the spray gun. The width of the spray



pattern must be adjusted to suit the areato be covered.

Figure 18 shows how continuity of motioncan be worked out for a flat surface suchas a table top. Figure 18 also shows adiagram of how continuity of motion maybe planned for solid objects such ascabinets, metal covers, cases, etc. In bothcases the spray gun is triggered tofeather out the strokes.

In any motion study for spray guns it isdesirable to determine the fewest numberof strokes and least amount of motionnecessary to obtain the desired coverage.Therefore, the design and use of suitablefixtures will be found a great aid.

Modifications to Technique

While the above techniques andproceedures were written withConventional Air Atomising spray guns inmind, they are all generally applicable tomost other types of spray equipment aswell. However, due to different fluidemission rates and atomisation methodsthere will be some differences as listedbelow:

HVLPGenerally, slightly decreased targetdistances (7 to 8 inches) will benecessary to counter the slower paintdroplet speed and the higher solventevaporation rate caused by the highervolumes of compressed air used by theseguns. It should not be necessary to slowdown pass speed and only in extremecases will solvent types need to change.Jetting distance will normally be less thanfor Conventional Air Atomising guns.

AirlessTarget distance will depend upon the tipsize and fluid flow used, although 8 to 12

inches is normally recommended.successfully because of low dropletspeed and lack of adjiustment of fan sizeon the gun. Pass speed will need to beincreased due to the increased fluid flowrates. Triggering is essential.

Airless Air AssistedSimilar to airless, although fluid flow rateswill be marginally less and targetdistances similar to Air Atomising gunsare recommended. Triggering is againessential.

Conventional Air AtomisedElectrostatic

As per non electrostatic air atomisingalthough target distance may need toslightly increase to get the best from theelectrostatic ‘wrap-round’ effect shown inFigure 19. Due to this and an increasedevaporation rate caused by theelectrostatic effect, solvent types may

Fig 19

need to be altered to slightly slowerblends. Faraday Cage effect will givedifficulty when trying to coat shieldedareas such as corners (Figure 20) wherepaint will be drawn toward the closestgrounded object. For the same reasoncomponents that are too close togetherwill tend to ‘rob’ paint from other areas.Care must be taken to avoid build up ofpaint on sharp edges and corners for thesame reasons. To help overcome thisproblem it may be necessary to reduce orturn off the electrostatic power and usethe gun like its non electroststaiccounterpart.



Airless Air Assist ElectrostaticAs per non electrostatic airless air assistalthough target distance may need toslightly alter to get the best from theelectrostatic ‘wrap-round’ effect.Increased

Fig 20.

evaporation rate will be worse due to theaddition of electrostatics to the airlessatomisation method. Faraday cage will beworse than Air Atomised equipment dueto lower paint droplet velocity, althoughwrap-around will also be increased.

Airless ElectrostaticAs per non electrostatic airless althoughtarget distance may need to slightly alterto get the best from the electrostatic‘wrap-round’ effect. Increasedevaporation rate will be worse due to theaddition of electrostatics to theatomisation method. Faraday cage will beworse than Air Assisted Airless equipmentdue to lower paint droplet velocity,although wrap-around will also beincreased.

Although only shown in diagrammatic form -what types of bad spraying technique doyou think caused the following bad spraypatterns and material deposition?

Ex 2.

Ex 1.



Ex 3.

ITW DeVilbissRingwood RoadBournemouthBH11 9LHTel: 01202 571111

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SPRAY GUN TECHNIQUE - spraytec.net Information/Spraying Techniques.pdf · SPRAY GUN TECHNIQUE PAGE 3 SPRAY GUN TECHNIQUE PAGE 3 with the work surface, causing excessive spray dust