Cleaning a suction-feed gun

First, loosen the gun from the paint container,

allowing the fluid tube to remain in the container,

and unscrew the air cap a few turns. Holding a

piece of cloth over the air cap, pull the trigger to

divert air down the fluid tube and drive the paint

back into the container. Next, empty paint from the

container and rinse it out with the appropriate solvent

and an old paint brush. Pour a small quantity of

the solvent into the container and spray it through

the gun to flush out the fluid passages. Remove the

air cap and immerse it in clean solvent, then dry it

out by blowing with compressed air.

If the holes in the air cap become blocked with

dried paint, a stiff brush moistened with solvent

will usually remove the obstruction. If not, a toothpick

or sharpened matchstick can be used to clean

out the hole. On no account must wire or a nail be

used, for this can distort the holes and permanently

damage the air cap, resulting in a distorted spray

pattern.

Cleaning a gravity-feed gun

Remove the cup lid, empty out surplus paint and

replace it with a small quantity of solvent. Replace

the lid, unscrew the air cap a few turns and, holding

a piece of cloth over the air cap, pull the trigger.

Air will be diverted into the fluid passage,

causing a boiling action and flushing it clean.

Spray solvent through the gun and clean the air

cap. Finally, with a solvent-soaked rag wipe the

outside of the paint container clean.

Cleaning a pressure-feed set-up

Shut off the air supply to the pressure tank, release

pressure in it and loosen the lid. Unscrew the air

cap a few turns, hold a piece of cloth over it and

pull the trigger. The pressure will force the paint

Figure 17.24Air cap, fluid tip and fluid needle

(DeVilbiss Automotive Refinishing Products)

Automotive finishing and refinishing 591

Figure 17.25Basic parts of a standard spray gun (DeVilbiss Automotive Refinishing Products)

1 Retaining ring for air cap

2 Air cap

3 Air cap and retaining ring

4 Corrosion resistant fluid tip and gasket

5 Baffle

6 Kit of five seals

7 Kit of five JGA-7 fluid needle packings

8 Fluid needle packing nut

9 Valve assembly

10 Kit of five circlips

11 Kit of five O-rings

12 Fluid needle

13 Spring

14 Kit of five gaskets

15 Gun body bushing

16 Fluid needle adjusting screw

17 Air valve assembly

18 Air valve

19 Kit of three springs

20 Kit of five screws

21 Kit of five trigger bearing studs

22 Locknut

23 Connector in BSP

24 Trigger

25 Air flow valve

26 Retaining ring

27 Connector in BSP

28 Baffle

from the gun and fluid hose back into the tank.

Empty surplus paint from the tank and pour in a

small quantity of solvent, replace the lid firmly and

pressurize the tank at about 0.3 bar (5 psi). The

pressure will force the solvent to the gun. Hold a

piece of cloth over the air cap and pull the trigger,

and the higher atomizing pressure will force the

solvent back into the pressure tank. If this is

repeated about a dozen times, the purging action

will clean out the fluid passages. Disconnect the

592Repair of Vehicle Bodies

fluid hose and blow it out with compressed air.

Steep and clean the air cap and finally dry out the

tank with a piece of cloth.

Whichever type of gun is used it must not be

immersed in solvent, as this causes the lubrication

oil to be washed away and will cause paint leakage

from the gun. In addition to this, the air passage

could become blocked with pigment sludge.

Lubrication

After the gun has been cleaned, a drop of oil

should be applied to the fluid needle packing, air

valve packing, and trigger fulcrum screw.

A spray gun will function efficiently provided

that it is clean and well maintained, but neglect

will eventually cause the gun to malfunction.

17.10 Spray gun motion study

The spray gun is not a difficult tool to master, but a

study of the following text and the accompanying

diagrams will be invaluable to the inexperienced

sprayer. Any person who is using a spray gun for

the first time should obviously spray a few practice

panels such as disused car doors, wings, bonnets

and so on to get the feel of the gun before undertaking

actual work.

Spraying flat surfaces

The gun should be held at right angles to the work

and at a distance of 150–200 mm. Should the gun be

held too close to the surface this will result in too

much paint being applied, causing runs and sags.

Holding the gun too far from the surface creates

excessive overspray and a sandy finish (Figure

17.26). The relationship of gun distance and stroke

When spraying a panel, the technique of ‘triggering’

the gun must be mastered. The stroke is

started off the panel and the trigger is pulled when

the gun reaches the edge of the panel. The trigger

is released at the other edge of the panel but the

stroke is carried on for a short distance before

reversing for the second stroke. This triggering

action must be practised and perfected to avoid a

build-up of paint at the panel edges and to reduce

paint wastage due to overspray.

The method of spraying a panel is shown in

Figure 17.26Spray distance Figure 17.29. Note that the gun is aimed at the top

Figure 17.27Maintaining spray distance

speed is easily understood, and with a little practice

the sprayer is able to adjust his speed of movement to

suit the distance between the gun and the surface.

The gun distance should be kept as constant as possible,

and arcing of the gun must be avoided to obtain

an even coating thickness. The correct gun action is

acquired by keeping the wrist flexible (Figure 17.27).

Do not tilt the gun; hold it perpendicular to the surface.

Tilting will give an uneven spray pattern resulting

in lines across the work (Figure 17.28).

Automotive finishing and refinishing 593

edge of the panel, and from then on the aiming

point is the bottom of each previous stroke. This

gives the 50 per cent overlap necessary to obtain a

wet coating. An alternative method is shown in

Figure 17.30. In this method the ends of the panel

are first sprayed with single vertical strokes, the

panel then being completed with horizontal

strokes. This technique reduces over-spray and

ensures complete coverage of the surface.

Long panels such as those encountered on furniture

vans require a different approach. A certain

amount of arcing is permitted to avoid a build-up of

paint where the strokes overlap, and the triggering

of the gun is very important. The length of each

horizontal stroke is 450–900 mm approximately, or

whatever the sprayer can manage comfortably.

Figure 17.31 shows the method of overlapping with

the panel being sprayed in separate sections,

each section overlapping the previous one by about

100 mm.

When spraying level surfaces such as car roofs

and bonnets, always start on the near side and

work to the far side to redissolve any overspray. A

certain amount of gun tilting is usually unavoidable

when reaching across a car roof and overspray

is thus created.

Spraying curved surfaces

As previously stated, the gun should be kept at

right angles to the surface and as near a constant

distance from it as possible (Figure 17.32).

Spraying external corners

Figure 17.33 shows the method of spraying the

edges and corners of a panel, the centre being

sprayed like a plain panel. Figure 17.34 shows the

technique used to paint vertical corners, the point

to watch being to half trigger the gun to avoid

applying too much paint.

Figure 17.28Tilting spray gun

Figure 17.30Alternative panel spraying method

Figure 17.31Spraying large panels

Figure 17.29Panel spraying method

594Repair of Vehicle Bodies

Figure 17.32Spraying curved surfaces

Figure 17.33Spraying external corners

Figure 17.34Spraying vertical corners

Figure 17.35Spraying internal corners

Figure 17.36Spraying internal corners: method for

better finish

strokes in order to avoid overspraying or double

coating the adjoining surface.

Spraying sequence

An automobile should be sprayed in sections,

spraying one section at a time before moving on to

the next one. Figure 17.37 shows a typical method,

but this may vary depending on the size and shape

Spraying internal corners

Spraying directly into a corner (Figure 17.35) gives

an uneven coating but is satisfactory for most

work. When an even coating is necessary, such as

with metallic finishes, it is better to spray each face

separately, starting with a vertical stroke at the

edge of the panel (Figure 17.36). The vertical

stroke should be followed with short horizontal

Automotive finishing and refinishing 595

of the vehicle concerned (Figures 17.38 and 17.39).

The painter must decide on his approach before

commencing to spray and then work methodically

round the car, finishing at a point where an overlap

is least noticeable. As the bonnet of the car is the

panel which attracts most attention by the customer,

most spray painters prefer to spray this last to avoid

the risk of overspray falling on it.

17.11 Spraying defects

No matter how excellent a spraying equipment is,

sooner or later some small trouble shows itself

which, if it were allowed to develop, would mar

the work done. However, this trouble can usually

be very quickly rectified if the operator knows

where to look for its source. The following sections

contain the causes and remedies of all the

troubles most commonly encountered in spraying.

Fluttering spray

Sometimes the gun will give a fluttering or jerky

spray (Figure 17.40), caused by an air leakage into

the paint supply line. This may be due to the following

(numbers correspond to those on the figure):

1 Insufficient paint in the cup or pressure feed tank

so that the end of the fluid tube is uncovered

2 Tilting the cup of a suction-feed gun at an

excessive angle so that the fluid tube does not

dip below the surface of the paint

3 Some obstruction in the fluid passageway

which must be removed

Figure 17.37Suggested sequence of spraying a car

Figure 17.39Spraying a vehicle wing (DeVilbiss

Automotive Refinishing Products)

Figure 17.38Spraying a vehicle roof (Racal Safety

Ltd )

596Repair of Vehicle Bodies

4 Fluid tube loose or cracked or resting on the

bottom of the paint container

5 A loose fluid tip on the spray gun

6 Too heavy a material for suction feed

7 A clogged air vent in the cup lid

8 Loose nut coupling the suction feed cup or

fluid hose to the spray gun

9 Loose fluid needle packing nut or dry packing.

Faulty spray patterns

The normal spray pattern produced by a correctly

adjusted spray gun is shown in Figure 17.41a, and

defective spray patterns can develop from the following

causes:

1 Top or bottom heavy pattern (Figure 17.41b)

caused by:

(a) Horn holes in air cap partially blocked.

(b) Obstruction on top or bottom of fluid tip.

(c) Dirt on air cap seat or fluid tip seat.

2 Heavy right or left side pattern (Figure 17.41c)

caused by:

(a) Right or left side horn hole in air cap partially

clogged

(b) Dirt on right or left side of fluid tip.

3 Heavy centre pattern (see Figure 17.41d)

caused by:

(a) Too low a setting of the spreader adjustment

valve on the gun

(b) Atomizing air pressure which is too low

or paint which is too thick

(c) With pressure feed, too high a fluid pressure

or a flow of paint which exceeds the

normal capacity of the air cap

(d) The wrong size fluid tip for the paint

being sprayed.

Figure 17.40Identification of parts of gun

responsible for fluttering spray

Figure 17.41Faulty spray patterns: (a) correct

pattern (b) top or bottom heavy (c) right or left sided

(d) heavy centred (e) split

Automotive finishing and refinishing 597

4 Split spray pattern (Figure 17.41e) caused by

the atomizing air and fluid flow not being properly

balanced.

To correct defects 1 and 2 (top or bottom heavy pattern,

or heavy right or left side pattern) determine

whether the obstruction is in the air cap by spraying

a test pattern; then rotate the air cap half a turn and

spray another test. If the defect is inverted the

obstruction is obviously in the air cap, which should

be cleaned as previously instructed. If the defect has

not changed its position, the obstruction is on the

fluid tip. When cleaning the fluid tip, check for fine

burr on the tip, which can be removed with P1200

wet-or-dry sandpaper. To rectify defects 3 and 4

(heavy centre pattern, or split spray pattern), if the

adjustments are unbalanced readjust the atomizing

air pressure, fluid pressure, and spray width control

setting until the correct pattern is obtained.

Spray fog

If there is an excessive mist or spray fog, it is

caused by:

1 Too thin a paint.

2 Over-atomization, due to using too high an

atomizing air pressure for the volume of paint

flowing.

3 Improper use of the gun, such as making incorrect

strokes or holding the gun too far from the

surface.

Paint leakage from gun

Paint leakage from the front of the spray gun is

caused by the fluid needle not seating properly

(Figure 17.42). This is due to the following (numbers

correspond to those on the figure):

1 Worn or damaged fluid tip or needle

2 Lumps of dried paint or dirt lodged in the fluid

tip

3 Fluid needle packing nut screwed up too tightly

4 Broken fluid needle spring

5 Wrong size needle for the fluid tip.

Faulty packing

Paint leakage from the fluid needle packing nut is

caused by a loose packing nut or dry fluid needle

packing. The packing can be lubricated with a drop

or two of light oil, but fitting new packing is

strongly advised. Tighten the packing nut with the

fingers only to prevent leakage but not so tight as

to bind the needle.

Air leakage from gun

Compressed air leakage from the front of the gun

(Figure 17.43) is caused by the following (numbers

correspond to those on figure):

Figure 17.42Fluid needle assembly

Figure 17.43Air valve assembly

1 Dirt on the air valve or air valve seating

2 Worn or damaged air valve or air valve seating

3 Broken air valve spring

4 Sticking valve stem due to lack of lubrication

5 Bent valve stem

6 Air valve packing nut screwed too tightly

7 Air valve gasket damaged.

Oil in air line

If the air compressor pumps oil into the air line, it

can have the following causes:

1 The strainer on the air intake is clogged with

dirt.

2 The intake valve is clogged.

3 There is too much oil in the crank case.

4 The piston rings are worn.

598Repair of Vehicle Bodies

Compressor overheating

An overheated air compressor is caused by:

1 No oil in the crankcase

2 Oil which is too heavy

3 Valves which are sticking, or dirty and covered

with carbon

4 Insufficient air circulating round an air-cooled

compressor due to it being placed too close to a

wall or in a confined space

5 Cylinder block and head being coated with a

thick deposit of paint or dirt

6 Air inlet strainer clogged.

17.12 Sanding and polishing machines

Sanding and polishing by hand can prove to be both

laborious and expensive. Unfortunately there are

many parts of vehicle surfaces where there is no

alternative but to carry out these processes without

the aid of power tools. However, surfaces such as

the roof, bonnet, boot lid and parts of the doors and

wings of cars can be rubbed down or polished more

economically and efficiently with power tools.

Damaged areas of paintwork can be rubbed down

very quickly with these machines, but final feather

edging is best done by hand. Two types of machine

are favoured by the refinishing painter, the rotary

sander and the orbital sander. Both are obtainable as

either compressed air operated or electrically driven

machines (Figures 17.44 and 17.45). In addition,

orbital sanders are available which operate with

compressed air and can be connected to a water supply

so that the worked surface is continually washed

with clean water whilst rubbing down takes place.

Sanding processes

In order to produce smooth, glossy finishes the

substrates and undercoats must be levelled down

without leaving deep scratches. Non-sand primer

surfaces have virtually eliminated many of the

problems associated with scratch swelling, but they

must be applied over staisfactorily prepared surfaces.

Where repair work has been carried out

which includes the use of polyester fillers and/or

surfacers, the repaired area must be carefully

sanded prior to applying the non-sand coatings.

The wet sanding of high-build undercoats and

fillers has long been the accepted method of levelling

these materials in order to avoid the creation

of excessive dust. In addition, this method of sanding

helps to clean the surface but also presents certain

problems. Water penetration behind window

and windscreen rubbers, door checks, etc. can lead

to lengthy drying-out times. All undercoats are

slightly porous and, if the moisture from the sanding

process is not completely dried out, problems

of micro-blistering and faulty adhesion may result.

The two main objections to dry sanding have

always been that the abrasive paper clogs up and

the process creates excessive dust in the workshop.

Figure 17.44Air driven orbital sander with dust

extraction (Desoutter Automotive Ltd )

Figure 17.45Electrically driven palm grip orbital

sander with dust extraction (Black and Decker Ltd )

Automotive finishing and refinishing 599

However, clogging up of the abrasive paper is no

longer a big problem since the introduction of

coated abrasives. These abrasive papers have a

coating over the grit particles, of a material which

is based on zinc stearate. This coating allows the

sanded-off residue to be shaken from the paper to

produce a fairly clean sanding surface, thus creating

more mileage from each sheet or disc of the

abrasive paper. Coated abrasive papers are available

in production paper grades (e.g. P40 to P120)

and also in finer grades of lubricoat papers (e.g.

P150 to P600).

It should be noted that, following sanding down

with these coated abrasives, the surface should be

cleaned with a proprietary spirit cleaner to remove

particles of the stearate coating from the surface

before applying paint. Failure to do so may result

in the appearance of ‘fish eyes’.

Dry sanding of painted surfaces, and surfaces to

be painted, has increased in popularity as a result

of the development of sanding tools which incorporate

dust extraction methods. These dust extractors

can be in the form of either a vacuum bag

attached to the sanding tool, or a large vacuum dust

collecting unit remote from the tool and to which

two or more sanders can be connected. An added

bonus with the remote machines is that they can

also be used as a vacuum cleaner for the workshop.

The sanding tools have a pad with eight or more

holes in them to which are attached sanding discs

with similar holes. In operation, the dust created by

sanding is drawn through the holes and deposited

in the collecting unit or bag.

Sanding machines available include types which

are dual acting: that is, they can be set to either

rotary or orbital (eccentric) actions (see Figures

17.46 and 17.47). The rotary action is in the region

of 420 rev/min, whilst the eccentric action moves

at 12 000 strokes/min. The machines can be either

electric or driven by compressed air. The rotary

action is suitable for removing old paint films and

surface rust and, when fitted with a polishing head,

the machine can be used for final polishing of

paintwork. The eccentric action is suitable for

feather edging, flatting surfaces prior to painting,

and levelling surfacers and fillers. The sanding

discs can be either self-adhesive (synthetic resin

adhesive) or of a type which has a looped velvet

reverse side which is simply pressed on to a special

pad. These pads themselves are available as hard,

soft and supersoft types depending on the type of

work for which they are required.

The Festo sanding systems for body repair and

paint shops can be of the mobile type which has the

extraction unit mounted on to castors (see Figure

17.48), or of the fixed position type. In the latter

type a boom, which can be moved over the car body,

has all the facilities that the worker requires, e.g.

compressed air supply hoses, extraction hoses, and

electrical supply for both 240 V and 110 V tools.

Figure 17.46Festo orbital sander (Minden

Industrial Ltd )

Figure 17.47Festo random orbital sander (Minden

Industrial Ltd )

600Repair of Vehicle Bodies

Polishing machines

There are many types of machine suitable for polishing,

and they all operate with a common circular

movement. To be suitable for polishing, however,

the machine must not rotate with too many revolutions

per minute as scorching of the film will easily

result. The pad is covered with a lambswool disc or

foam pad which must be kept clean at all times

(Figure 17.49). Remember that polishing is the

final process; should grit be picked up on the disc,

scratches will result which can ruin the finish on a

vehicle. Polishing machines are best used with liquid

polishes and should be used with a sweeping

movement. Even with slower revolving machines,

polishing in one spot for too long can cause

scorching and blistering of the paint film.

17.13 Preparation of a motor vehicle for

repainting

Two methods of preparing a vehicle for repainting

are open to the painter: (a) completely strip the

existing finish down to the metal substrate, or

(b) prepare the surface by rubbing down with abrasives.

The main point to consider when deciding

which process to use is the condition of the existing

finish allied to the extent of damage in the case of a

local repair. Which process to choose can only be

decided upon by a close inspection of the vehicle.

On a motor car that has been involved in a collision

and received damage to its front wing, but not

serious enough to warrant replacement, the panel

beater will carry out his work first, bringing the

wing as near as possible to its original shapes. The

vehicle then becomes the responsibility of the

painter, who will carry out a close inspection of the

condition of the paintwork. Should the paint show

signs of poor adhesion to the substrate, it is advisable

to strip it off completely using paint remover.

However, if the damaged area is not extensive and

the paintwork on the rest of the wing is in sound

condition, the work can be carried out, without the

use of paint remover, by rubbing down the area.