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 17.61 Short-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 manufacturers materials with another.
Although it is common practice in some workshops
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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 17.62 Infrared cassette, arch and paint shop (Infrarödteknik AB Stanners)
614 Repair of Vehicle Bodies
(b)
Figure 17.63 Application 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
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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 17.64 Burnishing 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 17.65 Rotary polisher fitted with a sponge
polishing pad (Faréncla Products Ltd)
616 Repair 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.
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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
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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.
618 Repair 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
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pressures of 58 bars (75120 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 manufacturers 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 manufacturers
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 17.66 Sealing wheel arches and floor pan
(Tuff Kote Dinol)
Figure 17.67 Sealing underbody (Tuff Kote Dinol)
620 Repair of Vehicle Bodies
Figure 17.70 Rapid 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 17.68 Comparison of cold spraying and hot
spraying
Figure 17.69 Typical viscosity curve
solvent, whereas in hot spraying the necessary
reduction in viscosity is achieved by heat. Heating
of enamels etc. to 6080 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 17.71 Suggested 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).
622 Repair 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 17.72 Lateral movement of vehicles using
two sets of tracks
Figure 17.74 Smaller bogies can be used with
three-track rails
Figure 17.73 Two-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.
624 Repair 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 17.75 Volumetric 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.
