Short-wave infrared paint drying

Infrared drying lamps have been used in finishing

shops for many years to force dry localized areas of

paintwork and to accelerate the curing of two-pack

polyester stoppers and fillers. A more recent development

is the short-wave infrared heating module

which, instead of easily breakable lamps, has heating

elements mounted into an aluminium casette.

One firm specializing in this type of equipment is

Infrarödteknik AB, whose IRT 100 unit has a reflector

coated with a thin layer of gold to give maximum

reflectivity and long life. In addition it is equipped

with a fan for solvent vapour removal, and a control

panel which guarantees efficient control of the drying

process according to the type of paint being used.

The unit is mounted on a stand with a flexible support

lever which makes it possible to locate the heating

module in any position without locking devices

(see Figure 17.61). A variation on this type of heater

is a vertical heater mounted on castors for force drying

of doors, quarter panels and so on.

Figure 17.61Short-wave infrared heater IRT 100

(Infrarödteknik AB – Stanners)

A further development is the infrared drying arch,

designed to be mounted inside an existing spray

booth. The arch consists of a number of heating

Automotive finishing and refinishing 613

modules mounted on to a steel frame in the form of

an arch. This arch is stored at the rear end of the

booth and is sheltered from overspray. Following the

spraying process, it is moved quickly to the vehicle

and then moves along the length of the vehicle at a

slower speed. After the stoving of the vehicle, it then

switches off automatically and moves back to the

rear of the booth (see Figure 17.62). Being computer

controlled, the arch is capable of providing the refinisher

with the option of drying a complete vehicle or

one or more panels at various positions on the vehicle

body. It also caters for the various types of paint

and colours.

Short-wave infrared units require no preheating,

which makes for fairly high savings where energy

costs are concerned. They also cool in seconds,

which means the vehicle can be moved out of the

booth almost immediately. When used to force dry

two-pack materials, the drying times claimed are

quite extraordinary: for example, a complete respray

can be dried in 5 to 10 minutes, and minor touch-ups

in 1 to 2 minutes. Single panels finished in a material

such as Bergers Standox 2K are baked in 4 to 5 minutes,

whilst primers and fillers bake even faster. With

drying times like this, the savings on energy costs

and the gain in turnover of work are obvious.

Two-pack paint system

There are many variations available to the refinisher

with these materials, and the paint manufacturers’

literature must be referred to. It is inadvisable to

mix one manufacturer’s materials with another.

Although it is common practice in some workshops

to use cellulose-based primer surfacers under

two-back finishes, better build and intercoat adhesion

is obtained when two-pack undercoats are

used. These undercoats are multipurpose materials

which can be used as primers or non-sand surfacers.

Selfetch 2K primer surfacers are available for

use on bare, prepared substrates such as steel, aluminium

and glass fiber, and can even be applied

over vehicle manufacturers’ finishes. Transparent

adhesion primers are available which can be

applied on to manufacturers’ finishes which have

merely been cleaned with a cleaning spirit and a

Scotchbrite pad. These primers can then be overcoated

with the finishing material after 15 minutes

as a wet-on-wet process. As previously stated, there

are many variations of materials with these products,

and so there is no basic system or process

which can be described. A variety of thinners are

available for use at various workshop temperatures.

Various hardeners may be used depending on workshop

temperature and humidity.

Two-pack paints are available as straight

colours, metallics and base-coat-and-clear systems.

To give some idea of the range of choice in twopack

base-coat-and-clear materials, the following

list will be helpful. Each of the clear coatings contains

an isocyanate hardener:

1 Base coat: acrylic/polyurethane synthetic

Clear coat: two pack (2K)

2 Base coat: acrylic synthetic

Clear coat: 2K modified acrylic synthetic

Figure 17.62Infrared cassette, arch and paint shop (Infrarödteknik AB – Stanners)

614Repair of Vehicle Bodies

(b)

Figure 17.63Application of water-based paints is

strictly controlled by the size of the fluid tip on the

spray gun

3 Base coat: acrylic lacquer

Clear coat: 2K modified acrylic synthetic

4 Base coat: cellulose

Clear coat: 2K acrylic synthetic or polyester/

acrylic.

It must be stressed that the time lapse between applying

the base coat and the clear coatings recommended

by the paint manufacturer must be strictly observed if

intercoat adhesion problems are to be avoided.

Water-based paint

Based paints use, as their name suggests, water

instead of solvent. The use of water means that the

emission of volatile organic compounds (VOCs) is

eliminated. VOCs generate photochemical oxidants

and suspended particulate matter (SPM); these are

sources of air pollution which most countries are

seeking ways to reduce.

Water-based paints are applied in the normal

way, but care must be taken with the volume of

paint delivered. The size of the fluid tip is therefore

very important (Figures 17.63a, b).

The use of water-based paint also allows the

reduction of carbon dioxide (CO2) by giving a

faster drying time. Toyota have developed a system

which does not need drying after the second coat,

the third coat is applied straight away. This also

saves painting time. This system reduces VOCs by

70% and CO2 by 15% compared to conventional

techniques. Water-based paint also offers easy

clean-up and is therefore better in terms of waste

management.

However, currently solvent clear lacquer coats

are still needed as the final finish needs to withstand

the elements of the weather.

17.15 Burnishing, polishing and final detail

work

Though brief reference to burnishing and polishing

has been made earlier, these subjects are important

enough to warrant fuller description. Even though

most car refinishing paints are nowadays formulated

to provide a good gloss from the gun, the

final appearance of the vehicle can be further

enhanced by careful burnishing and polishing.

Burnishing helps to smooth out the surface whilst

imparting a fuller gloss and revealing depth to the

colour (Figure 17.64). Polishing, if carefully done,

(a)

Automotive finishing and refinishing 615

nishing continued until a smooth glossy surface is

achieved. Some of the colour will come away on

the cloth but, provided that a sufficient coating

thickness has been applied, this is unimportant.

The cloths used for burnishing should be washed

out periodically in clean water to remove the

build-up of pigment which will hinder the action

of the cloth.

Machines can be used for burnishing large areas

of panel using liquid burnishing materials. These

liquids, which contain milder abrasives, reduce the

risk of swirls appearing on the surface as would be

the case should burnishing compounds be used with

machines. Hand burnishing, however, will produce

the better finish should costs permit. When using

machines for burnishing, the precautions set out in

Section 17.12.2 should be observed. Following the

burnishing process comes the final polishing.

Polishing

This is where the rotary-type polisher fitted with a

lambswool pad (Figure 17.65) comes into its own.

Figure 17.64Burnishing using rubbing compound

and a sponge polishing pad (Faréncla Products Ltd )

will improve the lustre still further and provide a

protective coating over the paint film.

Burnishing

Where the final coat of enamel contains small particles

of foreign matter, it is best flatted with P1200

grade wet-or-dry paper with soap as a lubricant.

The soap must be rubbed into the paper and not

into the painted surface. Care must be taken not to

rub through the colour coats, especially where projections

exist in the vehicle construction.

Following washing down, the car or van is now

ready for burnishing. The mutton cloth (or any

suitable soft cloth) is first wetted in clean water

and wrung out. A small quantity of burnishing

compound is applied to the cloth and rubbing can

commence, working in straight lines over a small

area. A fairly firm pressure should be applied at

first, but as the area shows sign of glossiness the

pressure should be reduced. The friction caused by

rubbing will generate heat, and when this takes

place the cloth must be turned over and the bur-

Figure 17.65Rotary polisher fitted with a sponge

polishing pad (Faréncla Products Ltd )

616Repair of Vehicle Bodies

These machines are best used with liquid car polishes,

but are also very effective when used with

the more viscous conventional polishes. The polish

is spread lightly over the surface with soft clean

cloth, following which the finish is brought up to a

high lustre with the machine. Heavy pressure is not

required when polishing with a machine. Some

parts of the vehicle body are obviously not suitable

for machine polishing and must be done by hand.

This is best done with soft yellow dusters rather

than mutton cloth, which can leave striations on

the surface and spoil the finished effect. The same

care and precautions should be practised when polishing

as when burnishing (see Section 17.12.2).

The importance of working cleanly cannot be overemphasized.

Final detail work

Attention to detail when completing a respray can

make the difference between a professionally done

job and the amateur one. The sequence of operations

is usually as follows:

1 Remove overspray from windows with a razorblade

scraper and wash leather.

2 Clean chrome bumper bars with rubbing compound.

3 Clean and refit all parts which have been

removed for the painting process.

4 Vacuum clean inside of car.

5 Paint tyres with tyre black. Rubber glazing

strips can also be painted with this material.

6 Finally carry out any lining work, using either

lining tape, fine lining brushes or roller-type

lining tools. The type of paint used is usually of

a synthetic resin nature.

17.16 Rust-proofing

A vehicle body contains many recessed areas and

box sections which are difficult, and in some cases

impossible, to paint properly with a spray gun and

conventional paints. The only time that they receive a

coat of paint is in the electropriming process carried

out by the vehicle manufacturer. All primers are

porous, and this material is no exception. Should

moisture be allowed to penetrate the primer, it will

eventually break down and corrosion of the substrate

will result. It is estimated that 90 per cent of corrosion

on vehicle bodies is of the ‘inside-out’ type.

In an effort to prolong the life of a car body and

provide an additional selling point, most manufacturers

now carry out a rust-proofing process as part of

the finished product or as a chargeable extra. A warranty

is provided to the customer, but usually on condition

that he or she has the vehicle checked for signs

of corrosion at regular intervals and is prepared to

pay for any rectification work that may be required.

The material used to treat these cavity areas is

based on wax dissolved in a solvent with rust

inhibitors added. A compound of this type has good

capillary attraction and can filtrate into joints, seams

and otherwise inaccessible areas. Different types of

wax coatings are available, some classed as penetrants

and others as heavy-duty coatings. There are

several firms worldwide who specialize in these

types of materials and the equipment for applying

them. One such company is Tuff Kote Dinol (TKD).

Measures employed by car manufacturers in an

effort to combat corrosion include the use of precoated

steel; washing and spraying the assembled

body; and immersion in anti-rust primer, which is

then baked. Spot-welded seams, which would be

prime sites for corrosion, are sealed; and wheelarches,

along with the underbody, are protected

with anti-chip coatings. After a further baking

period the car body is painted and some of the critical

cavities can be flooded with hot wax. However,

a car is subjected to a great deal of stress throughout

its life; it is scratched, scraped and banged, and

panels may flex, seams move and joints vibrate. If

untreated metal becomes exposed, rust will get in;

therefore cars need to be treated regularly, and so

rust-proofing becomes a continuous process.

The basic anti-corrosion tool is the spray gun

with its four attachments. The rigid and flexible

lances are best for all enclosed areas; the hook

nozzle is designed for more accurate, directional

work; while the fan spray tool is for coating open

underbody surfaces.

Spray nozzles

Rigid lance (1100 mm)

This produces a 360 degrees spherical spray pattern

at right angles to the lance, combined with a

forward and backward directed spray that allows

all surfaces – front, back, sides, top and bottom –

to be coated in one single sweep or stroke. It is

highly effective in places where straight structures

Automotive finishing and refinishing 617

such as doors, tailgate panels and long channels

exist, or where the operator needs to control the

position of the lance, such as enclosed front or rear

wing box sections.

It is advisable to place the tip of the lance into

the section being processed and to fog spray on

both inward and outward strokes. Do not force into

access holes. Test penetration before spraying to

ensure adequate clearance for lance.

Flexible lance (1100 mm)

This is basically a flexible version of the rigid lance.

It produces a 360 degrees spherical spray at right

angles to the lance combined with a straight-ahead

jet. It is highly effective in long narrow sections

which would otherwise be inaccessible or awkwardly

positioned and where its flexibility enables it

to operate even where there are bends and restrictions

which would prohibit the use of the rigid lance.

The main use of the flexible lance is for treating

sills, underbody box sections, strengtheners and

pillars. Its limitations are in narrow sections where

it cannot gain entry or in very large box sections

where it is desirable to control the position of the

tip of the lances, such as in doors.

It is advisable to work the lance into the desired

position and to fog on the outward stroke. In larger

sections, retract the lance slowly so that one sweep

will sufficiently cover all surfaces.

To extend the life of the nylon tube, the sides of

the lance should be held away from the edges of

the hole as it is entered or withdrawn. To eliminate

snagging on the edge, always hold the exposed part

of the lance at right angles to the hole. Feed the

lance into the section gradually, keeping thumb

and forefinger around the tube, close to the hole.

Use the same technique when withdrawing the

lance. The tube will go slack just before the tip

emerges from the opening. This reduces the risk of

accidentally spraying the interior trim and fittings

by withdrawing the lance too rapidly.

Hook nozzle

This produces a highly atomized forward-directed

full-cone jet which gives a powerful long range and,

at the same time, good dispersion. Its value lies in

its long range and directional capability, so that the

product can be directed precisely where pointed.

This makes it suitable for treating narrow box

sections such as door pillars, boot and bonnet lid

reinforcements, and areas with difficult access

such as door hinge areas, head and tail lamp housings,

wing supports, door sills and underbody box

sections, reinforcements, and suspension mountings.

It may also be used for spray coating the

wheel arches and underside of the vehicle.

The thumb should always be positioned on the

machined flat portion of the neck to direct the nozzle

where required. When spraying into narrow

sections, the nozzle should enter the section with

the spray directed towards the surface immediately

opposite the opening into the section. As soon as

the trigger is applied, the nozzle should be moved

in an arc until the spray is being directed towards

the end of the section. This should be repeated in

the opposite direction. Where seam penetration is

required, the nozzle should be specifically directed

toward the seam.

Straight nozzle

This produces a forward spray with a restricted

spray width. Ideally this nozzle should only be used

with underbody sealants on to easily accessible panels.

More obscure areas such as ledges within wheel

arches should be processed with the hook nozzle.

Rules for nozzle selection

To minimize operator error when processing

enclosed sections, always use the flexible or rigid

lances first. Only use the hook nozzle where openings

are insufficient for access with the 360 degree

tooling. An exception would be where a very narrow

section is to be processed and it would be

preferable to spray with a powerful directional jet,

such as a bonnet or boot lid strengthener.

Materials

Various anti-rust corrosion compounds include

heavy-duty waxes and sealants for wings, wheel

arches and underneath the vehicles; highperformance

cavity waxes and penetrants for all

enclosed areas, box sections, doors and pillars; special

engine compartment waxes; and waxes for special

purposes. These compounds are waxes and

inhibitors dissolved in a solvent. They are applied as

a liquid, but solidify as the solvent evaporates. For

ease of application make sure the materials and the

vehicle to be treated are at room temperature (about

15 °C). This is particularly important in winter.

618Repair of Vehicle Bodies

Process

To assemble the gun, put a can of wax into the

pressure pot, connect the head to the air supply,

and regulate the pressure. In general, work with

pressures of 5–8 bars (75–120 psi). Attach the

appropriate tool and then spray. To change materials,

release the pressure and replace the can. Try

not to let the gun get too dirty because, although it

is robust, dust and dirt will clog the nozzle.

The principle of rust-proofing is to prevent the

atmosphere and corrosive substances from attacking

the metal of the car body by applying an

impermeable layer to the metal. In this case it is a

layer of a penetrant or a sealant. Applying these

materials is straightforward enough, but it does

need care and attention.

Engine compartment

When protecting the engine compartment, the only

preparation necessary is thoroughly to clean and

dry the engine unit and compartment. Assemble the

gun and load the engine compartment wax, which

is designed to withstand high temperatures. The

hook nozzle is best here, as the spray from the flexible

lance would go everywhere and cover everything

with wax. A face mask is necessary when

working in confined areas or on open surfaces.

Putting your thumb on the flat section at the

base of the hook will help to direct the spray.

Beware of one piece of metal getting in the way of

another, and by obscuring it prevent it from getting

fully coated. The gun is two stage: the first pressure

delivers just air, and the second delivers both

air and material. The quantity of material is proportional

to the distance you pull back the trigger.

Match the speed of your sweep to the amount of

spray to avoid build-up of wax. Work in short

bursts rather than spraying continuously, and

always remember to hold the gun upright.

Using the flexible lance, spray inside the front

cross member and any other reinforcing sections.

Unprotected cavities are prone to rust. Watch out for

the join between sheets of metal, as dirt and moisture

can gather here and start rust. Use the hook spray,

since it is directional. Always wipe away any surplus

anti-corrosion fluid as you go. Make sure you do

inside all the reinforcements. Spray with reduced

pressure, otherwise anti-corrosion fluid will go

everywhere. Work from the bottom up, so that anticorrosion

fluid will not drip on you. When you rustproof

the chassis legs you should use a penetrating

fluid and the flexible lance for all enclosed areas.

Pillars

Whenever possible remove the courtesy input switch

for access, otherwise extra holes may have to be

made. Spray as far up the pillar as possible, then

down to the sills. Finally extend and secure the webbing

on inertia reel seatbelts and spray the surrounding

areas with great care. Spray into the D-post. Pay

attention to fully coating the wheel arch seam area.

Door panels and rear quarter panels

Use the existing openings to spray into door panels,

or drill extra access holes as required. Take care to

thoroughly protect all door hinges. Remove the trim

where necessary, to gain access to the rear quarter

panel. Reach over the rear wheel arch and spray coat

the wheel arch seam, the lower rear wing, around

the tail lights and into all the inaccessible areas.

Remove the existing plugs for access to the tailgate

panel. Spray up beside the window of the tailgate; if

unprotected this is particularly vulnerable to rust.

Sill panels and underbody sections

It is most important to achieve thorough coverage of

the sills. Use the manufacturer’s existing access

hole, or open extra access holes. Look out for double

sill sections, which must both be treated. Treat

the underbody box sections and cavities in a systematic

manner. Underbody seams, joints, brackets and

attachments must all be coated with the fluid.

Thoroughly apply a coating of rust-proofing fluid to

the underside of the car, paying special attention to

exposed seams and joints well away from the spray.

Wheel arches and underbody

Before applying a coat of sealant or heavy-duty

wax, first remove any loose or flaking materials.

When spraying, avoid blocking the manufacturer’s

drain holes; they are a vital part of anit-rust protection.

If they do get blocked, clear out the surplus.

The wheel arches must be done carefully

(Figure 17.66). Apply an extra coating to all forward-

facing surfaces. To give the best possible abrasion

resistance, pay particular attention to all joints

and recesses. These are the places where mud can

easily accumulate. When spraying the chassis legs

try not to get any overspray in the engine compartment.

Using the hook nozzle, deposit a generous

Automotive finishing and refinishing 619

coating of sealant all the way round. Make sure that

the ledges are fully coated; not all of these are visible

from underneath, so check them when the vehicle

is back on the ground.

Spray the underbody from every direction to

prevent shadow areas (Figure 17.67). Check your

work as you go. Use the fan spray nozzle for rapid

coverage of large open areas. Use the hook nozzle

to get a good finish around the edges and to reach

into awkward corners, then wipe away any excess

sealant before it sets.

Clean any excess rust-proofing fluids off the car

body with the recommended solvent, and remember

to always clean the gun thoroughly after using

the sealant or other heavy-bodied products.

In some situations you must rust-proof: for

instance, when fitting a new body panel which is

supplied untreated or when you do service bodywork

repairs. But rust can also develop as a result

of any accidents. Check the vehicle when it comes

in for signs of external corrosion or paint damage;

Figure 17.66Sealing wheel arches and floor pan

(Tuff Kote Dinol )

Figure 17.67Sealing underbody (Tuff Kote Dinol )

620Repair of Vehicle Bodies

Figure 17.70Rapid drop in paint temperature from

fluid tip to job surface

if neglected it risks forfeiting the protection of any

body warranty given.

17.17 Comparison of hot and cold

spraying

The true value of a paint film is the amount and

character of the solids deposited that remain after

drying or curing. Generally it is not possible to

apply paint by spray as supplied by the paint manufacturers.

To obtain the necessary consistency for

spray application and good flow-out, thinners are

normally used. The function of a solvent is merely

to reduce the viscosity of the paint to assist its

application to the work.

A paint at a viscosity of 35 seconds BS B4 flow

cup for conventional spray application requires a

high air pressure to produce good atomization

(Figure 17.68). The high pressure and the expanding

compressed air evaporate a lot of the solvent

and carry off some of the finely atomized paint as

overspray. This is a recognized loss, and therefore

less solids are deposited per coat. The basic principle

of hot spray is to reduce the initial viscosity of

the paint by heat, in contrast to cold spray where

the viscosity is reduced by the addition of a thinner

which subsequently evaporates. The underlying

principle of a spray system involves splitting up the

fluid paint into tiny droplets. This atomization is

only possible if the viscosity of the liquid is sufficiently

low. In conventional cold spray application

this low viscosity is achieved by the addition of a

The viscosity of paints can vary in the paint shop

owing to changes in the atmospheric and shop temperatures

throughout the working day. This can be

a problem to the painter, as work can be subject to

uneven build and runs and sags can occur in spite

of gun adjustments. Even if wide temperature

changes do occur in hot spray, the atomization and

spray pattern is unaffected because the paint heater

delivers the fluid at a controlled temperature and

therefore the viscosity remains constant. At higher

temperatures temperature viscosity is negligible,

but there is a sharp variation within normal atmospheric

temperature range (Figure 17.70).

Figure 17.68Comparison of cold spraying and hot

spraying

Figure 17.69Typical viscosity curve

solvent, whereas in hot spraying the necessary

reduction in viscosity is achieved by heat. Heating

of enamels etc. to 60–80 °C generally results in the

viscosity becoming one-third to one-quarter that at

ambient temperature. Further heating does not usually

cause any further fall (Figure 17.69).

Automotive finishing and refinishing 621

In hot spray application, low boiling solvents are

released more quickly during atomization because

of the heat. These are therefore not present in the

applied liquid film to cause chilling and resulting

condensation, called ‘blushing’, which occurs with

certain paints on applications under humid conditions.

The effect is further checked because the

material does not reach the low temperature when

chilled by the evaporating solvent at the spray tip

of the gun.

17.18 Movement of vehicle in the paint

shop

In any refinishing shop, the movement of vehicles

is of primary importance. Much time and effort

may be wasted in moving vehicles from one point

to another as dictated by the finishing process. This

is particularly true in those shops where expensive

air conditioned spray booth and/or spray booth

ovens have been installed. To recoup the high capital

outlay in such units it is essential that they

should be used at full capacity for the whole of the

working day. The simplest spraying/stoving unit,

i.e. the combined spray booth and oven, is capable

of handling six to seven vehicles per working day

of eight hours, providing it is kept in use over the

lunch hour. Furthermore, with soaring building

costs and high rateable values, coupled in some

cases with restricted space for expansion, it is

important that floor space be kept to a minimum.

Whilst it is not possible to achieve a factory-type

flow line for refinishing jobs that may vary tremendously

in their requirements, e.g. size, extent of

repair and amount of work to be carried out, the

desired continuous supply of vehicles to the spray

booth may be achieved. Careful consideration of

the problems of the vehicle movement will enable

this supply to be obtained with the minimum of

time, labour and space.

There are several methods of moving vehicles

within the refinish shop:

Under own power This method is the most economical

of labour, but requires the maximum of

space. Also it gives rise to additional dirt, air pollution

and fire hazards.

Manually Space requirements are as above but

demand on labour is heavier. Three men may be

required to move an average car.

Mobile hydraulic jacks These permit some saving

of space in that they allow a tight turning circle

both front and rear.

Turntable This gives an excellent means of utilizing

what would otherwise be a dead corner, or may

permit the spray booth to be sited in an otherwise

impossible position (Figure 17.71).

Figure 17.71Suggested use of turntable

Rail and bogie system The vehicle is moved

along railway-like tracks with bogies under

each of its four wheels. This offers the greatest

advantages of all sideways movement of the vehicles.

Normally four sets of parallel rails are

involved. The first pair constitutes the track on

which the vehicles are first received into the paint

shop, and along which the preparatory work is

usually carried out, i.e. discing, flatting and stopping.

The second pair is kept clear to allow movement

of selected bodies to the spraying and/or

stoving position (Figure 17.72). Vehicles are

either driven or manhandled from one track to

another. It is possible to have a layout utilizing

only one track, but movements are more restricted

than with two tracks. Two types of track are available:

(a) those with sets of two rails, and (b) those

with sets of three rails. With type (a) bogies long

enough to accommodate vehicles of differing

wheel bases must be used (Figure 17.73). Type

(b) permits the use of smaller, lighter bogies

which are on selected pairs of rails from each set

(Figure 17.74).

622Repair of Vehicle Bodies

17.19 Common spray painting defects

Blistering

Blisters are hemispherical projections on the

painted surface, which will contain either air or

water. Blisters can appear on surfaces which have

been freshly painted or which have been painted

for some time. Some of the causes of blistering

are:

1 Moisture on the surface prior to painting

2 Excessive heat on article during service

3 Salts on surface prior to painting.

The third cause can lead to the defect known as

‘contamination blistering’, which appears mainly

on those panels of a vehicle concealing the engine,

and which are subject to fluctuating temperatures.

It can be avoided by using demineralized water for

the final rinse prior to painting. Blistering can

occur on old paint films where the metal substrate

has corroded from the reverse side, e.g. car wings,

sills and lower door edges. Moisture penetrates

through the metal to the paint film and causes it to

swell at localized spots.

Blooming

This defect can be slight or severe. A slight case of

blooming appears as a surface haze on a freshly

dried paint film caused by moisture precipitation.

It can be easily removed with a damp washleather.

If moisture is deposited on to a tacky paint film it

can be absorbed by the binder or medium, which

will swell and give a cloudy, milky appearance

with loss of gloss. This type of bloom cannot normally

be cured, except by flatting and recoating.

Where there is a high degree of humidity in the

paint shop, it is best to use high-boiling (slow)

thinner to avoid blooming.

Blushing

Blushing is similar in appearance to blooming but

without loss of gloss. It is confined to wet paint

films which are subjected to moisture precipitation,

forming an emulsion with the binder. Blushing

appears mainly with lacquers and can usually be

cured by overspraying with a mist coat of antiblush

solvent.

Figure 17.72Lateral movement of vehicles using

two sets of tracks

Figure 17.74Smaller bogies can be used with

three-track rails

Figure 17.73Two-track rails requiring long bogies

Automotive finishing and refinishing 623

Bridging

This occurs when the paint film contracts too much

on drying so that it pulls away from recesses and corners.

Too fast a thinner and too heavy a coating will

produce this defect. Nitrocellulose lacquers particularly

tend to exhibit this type of behaviour. Should

the defect be discovered shortly after it has occurred,

it can often be cured by slitting with a sharp knife or

razor blade and overspraying with a slow solvent, or

by adding slow solvent to any subsequent coats.

Cobwebbing

This is a defect of faulty atomization in which the

paint leaves the spray gun in strings rather than

small atomized particles. It is caused by incorrect

thinning and is quite common in the spraying of

acrylic enamels. When cobwebbing occurs, spraying

should be stopped and solvent added to the

paint. Though undesirable in the normal painting

of motor vehicles, cobwebbing can be used to

advantage to obtain decorative effects.

Dry spray

Though dry spraying can be an advantage to the

spray painter, it is more often regarded as an undesirable

defect as it gives the work surface a sandy

appearance. It is caused by:

1 Too high an air pressure

2 Using solvents which are too fast

3 Moving the gun too rapidly or too far from the

job

4 Too high a paint viscosity

5 In hot spraying, too high a paint temperature or

surrounding air too hot.

Excessive overspray

Overspray can give a dry spray appearance and is

invariably the result of faulty spray gun manipulation.

However, some articles are so shaped that

oversprayed areas are difficult to avoid. The spray

painter should attempt to keep these areas ahead of

the gun, so that with subsequent passes of the gun

he can coat over the dry spray and dissolve it into

the wet paint film. When this is not possible, overspray

can be redissolved by mist coating the

affected areas with a high-boiling (slow drying)

thinner, or by flatting and burnishing.

Lifting

In this defect the existing paint on a surface wrinkles

and then can be scraped or peeled off following

the application of a fresh coating on to it. It is

usually caused by the action of the solvent in the

top coating attacking and softening the binder of

the previous coat, such as when cellulose materials

are applied over synthetic paints. When lifting

occurs there is little alternative but to strip and

recoat.

Orange peel

This is the most common of all spray painting

defects. As the name implies, the dried paint film

has the pebbly appearance of an orange skin, and

is caused by the failure of the wet paint film to

flow out smoothly after application. This defect

can be caused by any, or all, of the following

reasons:

1 Paint of too high a viscosity (too thick)

2 Too fast a thinner (thinner evaporating too

quickly)

3 Too low an air pressure, causing poor atomization

4 Too high a temperature in the spray room,

causing the paint film to ‘set up’ before it can

flow out

5 A sudden draught in the spray room (also causing

the paint film to set up prematurely)

6 In hot spraying, too high a paint temperature.

Though orange peel is a defect of faulty flow-out,

it can occasionally occur after the paint film has

flowed out. This is the result of uneven solvent

evaporation and the formation of vortices in the

film, caused by improper solvent balance.

Pin-holing

This is the sudden appearance of small vortices

(pin-holes) over the painted surface during drying.

It is caused by using a solvent in the paint

mixture which is too slow to evaporate, and

occurs when the topmost skin of the paint film

has dried out. The slow solvent is trapped beneath

the skin and ruptures it when escaping. Pin-holding

can also be caused by too high a room temperature,

and in hot spraying too high a fluid

temperature.

624Repair of Vehicle Bodies

Runs, sags, curtains

Each of these defects is the result of applying too

heavy a coating thickness. Some of the factors

which contribute to this are:

1 Paint which is too thin or too thick

2 Paint solvent which is too slow, retarding drying

3 Spray room temperature too low, also retarding

drying

4 Spray gun held too close to the surface, or

spray gun movement too slow

5 Fluid nozzle too large for material being sprayed.

Shelving

This term, though not widely used, is an

extremely apt one to describe the defect in which

the paint films tend to lie in separate layers or

shelves one on top of the other. This is caused by

failure of the top coat to adhere properly to those

previously applied, so that one coat can be separated

from the other by scratching with the fingernail

or a knife. Shelving usually shows during

sanding and can seldom be repaired. It is better to

strip and refinish a surface rather than risk any

subsequent coatings peeling off. The main causes

of shelving are:

1 The solvent in the top coat is too weak.

2 The first coat is too dry before the second coat

is applied.

3 There is grease or dirt between coats.

4 The top coat is too brittle.

5 There is poor natural adhesion in the top coat.

17.20 Colour mixing and matching

Colour mixing systems

Surveys have shown that, between them, 40 car

manufacturers offer approximately 600 colours on

current models. These are new colours only, and

the total number of colours that the refinisher may

have to match can run into thousands.

The development of a new car colour can be

broken down into four stages:

1 Initiated by clothing and other fashions, new

colours are reproduced in a range of paints. The

car body stylist then selects a colour by trial

and error, to suit a new car model.

2 Paint manufacturers then reproduce this colour

and carry out the usual tests for colour stability,

durability, etc.

3 The vehicle manufacturer paints a number of car

bodies with the colour to ensure that the paint is

suitable for mass production application.

4 Finally, the paint manufacturers are required to

produce a coloured paint to suit the vehicle

refinisher and to provide colour mixing formulae

as well as basic colours and tinters.

There are two colour mixing systems available to

the refinisher:

Gravimetric This system involves the use of a set

of scales. The base colour and tinters are mixed

according to their weight, the quantity of each

being obtained from a given formula.

Volumetric The various quantities of each ingredient

are added by volume. An adjustable measuring

rod is the main tool involved (see Figure 17.75).

Figure 17.75Volumetric colour mixing machine

In both systems the materials required to mix and

match colours are base colours, full-strength tinters,

and reduced-strength tinters. The last are necessary

in order to provide the refinisher with a measurable

quantity. For example, instead of adding 0.5 grams

of blue-black full-strength tinter, he can more easily

add 5 grams of reduced-strength tinter.

A complete set of colour formulae is provided

by the paint supplier and is updated at regular

intervals. Colour samples are also provided which

Automotive finishing and refinishing 625

are added to as new colours are produced. There

can be several slight variations of a colour as vehicles

leave the production line, and these can provide

the refinisher with real problems when trying

to match a colour. In order to assist with solving

these problems, the paint manufacturers produce

samples of these colour variants and literature with

hints on how to match them.

Colour mixing systems should not be installed in

the workshop if at all possible. They constitute a

fire hazard, and can collect dust and overspray

which may affect the accuracy of the scales. A separate

colour mixing room should be provided

where possible, which has plenty of natural light

and also has ‘daylight’ fluorescent lighting.

It should be fully understood that the vehicle

manufacturer uses high-bake materials, whereas

the refinisher cannot. In order to match a colour,

different pigments may have to be used in the

refinishing material in order for them to be compatible

with the binder. Consequently, a perfect

match may be obtained in daylight conditions, but

the colour may alter drastically when viewed

under sodium or mercury street lights. There is

nothing that the refinisher can do about this, and it

is advisable to explain this to the customer when

only part of the car, such as a wing or door, is to

be painted.

The modern system of supplying the refinisher

with colour formulae is in the form of microfiches,

which require a microfiche viewer or reader.