Poor adhesion of the gel coat
Resin
Unless the adhesion of the gel coat to the backing
laminate is very poor, this defect will be
noticed only when structure is being handled and
pieces of gel coat flake off. Areas of poor adhesion
can be detected sometimes by the presence
of a blister, or by local undulations in the surface
when it is viewed obliquely. Poor gel coat adhesion
can be caused by inadequate consolidation
of the laminate; by contamination of the gel coat
before the glass fibre is laid up; or, more generally,
by the gel coat being left to cure for too
long (Figure 16.48).
Figure 16.48Poor adhesion of the gel coat (Scott
Bader Co. Ltd )
570Repair of Vehicle Bodies
Figure 16.50Severe blisters (Scott Bader Co. Ltd )
insufficient time had been allowed for the mat to
absorb the resin before rolling. Blisters of this
kind can usually be detected by inspection as
soon as the moulding has been removed from the
mould (Figure 16.50).
Figure 16.49Fibre pattern (Scott Bader Co. Ltd )
Fish eyes
On a very highly polished mould, particularly when
silicone modified waxes are used, the gel coat is
almost non-existent. This shows up as patches of
pale colour usually up to 6 mm in diameter. It can
also occur in long straight lines following the
strokes of the brush during application. This fault is
rarely experienced when a PVAL film is correctly
applied.
Blisters
The presence of blisters indicates that there is
delamination within the moulding and that the air
or solvent has been entrapped. Blisters which
extend over a considerable area may also indicate
that the resin is undercured, and this type of blister
may not form until some months after moulding.
Blisters can also occur if the moulding is
subjected to an excessive amount of radiant heat
during cure. A possible cause of this defect is the
use of MEKP rather than cyclohexanone peroxide
paste. If, on the other hand, the blister is below
the surface, the cause is likely to be imperfect
wetting of the glass fibre by the resin during
impregnation. This would be due to the fact that
Crazing
Crazing can occur immediately after manufacture
or it may take some months to develop. It appears
as fine hair cracks in the surface of the resin.
Often the only initial evidence of crazing is that
the resin has lost its surface gloss. Crazing is generally
associated with resin-rich areas and is
caused by the use of an unsuitable resin or resin
formulation in the gel coat. The addition of extra
styrene to the gel coat resin is a common cause.
Alternatively the gel coat resin may be too hard
with respect to its thickness. In other words, the
thicker the gel coat the more resilient the resin
needs to be. Crazing which appears after some
months of exposure to the weather or chemical
attack is caused by either undercure, the use of too
much filler, or the use of a resin which has been
made too flexible (Figure 16.51).
Star cracking
This is the result of having an over-thick gel coat,
and occurs when the laminate has received a
reverse impact. Gel coats should never be more
than 0.5 mm thick (Figure 16.52).
Fibre pattern
The pattern of the composite reinforcement is
sometimes visible through the gel coat or prominently
noticeable on its surface. This usually
occurs when the gel coat is too thin or when the
reinforcement has been laid up and rolled before
the gel coat has hardened sufficiently, or when the
moulding is removed too early from the mould
(Figure 16.49).
Reinforced composite materials 571
has not been adequately cured, or that it is an
unsatisfactory resin for that particular application.
Cracking, shrinking and
Discolouring
The identification of this fault is visual. Deep
cracks appear in the resin and the colour changes
from a green to a mauve purple and hot to the
touch. The cause of this fault can be large build-up
of resin due to drainage or to excess application;
both lead to extreme exotherm. Alternatively, incorrect
amounts of catalyst or accelerator may be used,
usually an excess. The effect of the build-up of
exotherm is to cause excessive shrinking of the
moulding and internal stress. The extreme exotherm
can also damage and distort the mould itself. This
can be prevented by: avoiding large amounts of
resin (but where this is necessary the build-up
should be gradual); using the correct amount of
ingredients; being aware of any increase in workshop
temperatures; and noting any variations in the
correct percentages of catalyst or accelerator used.
Low-rigidity laminate
The first identification is by touch; the laminate will
feel spongy and more flexible than usual.
Alternatively, check by applying a Barcol hardness
tester. The common causes of this fault are low resin
content or undercure of the resin. To prevent this
fault, ensure the correct ratio of resin to glass and
eliminate any draughts. Large areas should be made
to gel quicker to cut down styrene loss, and correct
proportions of catalyst and accelerator must be used.
Figure 16.52Star cracks (Scott Bader Co. Ltd )
Figure 16.53Internal dry patch (Scott Bader Co. Ltd )
Figure 16.51Crazing (Scott Bader Co. Ltd )
Internal dry patches
These can be caused by attempting to impregnate
more than one layer of mat at a time. The presence
of internal dry patches can be readily confirmed by
tapping the surface with a coin (Figure 16.53).
Leaching
This is a serious fault. Leaching occurs after
exposure of the laminate to the weather, and is
characterized by a loss of resin from the laminate,
leaving the glass fibres exposed to attack by moisture.
Leaching indicates either that the resin used
572Repair of Vehicle Bodies
16.11 Safety precautions
The handling of polyester resin, glass fibre, and
ancillary materials such as catalysts, presents several
hazards which can be reduced to a minimum
if the correct precautions are taken. Most glass
fibre materials and resins are perfectly safe to use
provided the potential hazards are recognized and
reasonable precautions are adopted. Normally you
will have no problems if you follow these rules:
1 Do not let any materials come into contact with
the skin, eyes or mouth.
2 Do not inhale mist or vapours, and always work
in a well ventilated workshop.
3 Do not smoke or use naked flames in the
workshop.
Storage precautions
Liquid polyester resins are flammable but not highly
flammable, most of them having a flashpoint of
31 °C. Resins and accelerators should preferably be
kept in a brick-built store conforming to the normal
fire regulations for a paint store. The storage life of
polyester resin is about six to twelve months provided
the resin is kept below 20 °C in the dark (in metal
drums). Storage at higher temperatures, even for only
a few days, will considerably reduce the shelf life.
Catalysts are organic peroxides and present a
special fire hazard. They should be stored in a separate
area, preferably in a well ventilated fireresisting
compartment. If kept reasonably cool they
will not burn or explode. In case of fire in the
vicinity, they can be kept safe by drenching the
containers with water.
Operating precautions
Most polyester resins contain monomeric styrene,
which is a good grease solvent and may cause irritation
to the skin. The most effective method of
protecting hands is the use of a barrier cream or
rubber gloves, and this is strongly recommended.
Resin can be removed from the hands with proprietary
resin removing creams, or with acetone followed
immediately by a wash in warm soapy water.
In sufficient concentration styrene vapour is irritating
to the eyes and respiratory passages, and therefore
workshops should be well ventilated. When
resin is sprayed a gauze mask should be worn to
protect the mouth and nose. This also applies to
trimming operations when resin and glass dust can
cause irritation.
Catalysts are extremely irritating to the skin and
can cause burns if not washed off immediately
with plenty of warm water. Particular care must be
taken with liquid catalysts to avoid splashing,
spilling or contact with the eyes. Protective goggles
should be worn as a necessary precaution.
Workshop conditions
The building should not be damp and it should be
adequately heated and ventilated. Good head room is
desirable and sufficient space should be allowed for
all operations. The floor area should be divided into
sections as follows: preparation of reinforcement,
mixing of resins, moulding, trimming and finishing.
Resin and curing agents should be stored away from
the working area in a cool place, observing the necessary
precautions for flammable liquids and keeping
in mind the special hazards associated with organic
peroxides. Glass fibre should be stored and tailored
under dry conditions and separately from the moulding
area. The temperature of the building should be
controlled between 15 °C and 25 °C. Ventilation
should be good by normal standards, but draughts
and fluctuations in temperature must be avoided, so
doors and windows should not be used for ventilation
control. Dust extraction in the trimming section
should be of the down-draught type. Cleanliness is
important both for health of the operators and for
preventing contamination of resin and reinforcement.
As far as possible, health and comfort depend in
the first place on planned extraction, and in the
second place on workshop education in the nature
of the materials used. Almost all the offence comes
from the styrene vapour and glass filaments, both
of which advertise their presence before the concentration
reaches a danger level. As far as is
known the only real source of physical harm is the
dust produced in grinding, but all the materials and
byproducts contribute to discomfort, and sensible
evasive action is essential.
Spillage and disposal
Most of the following products are covered by the
terms of the Deposit of Poisonous Wastes Act:
Polyester resin Absorb spillages in dry sand and
dispose by landfill or controlled incineration.
Reinforced composite materials 573
Furane resin Extinguish all naked lights, open
doors and windows. Absorb spillage into sand or
chalk. Pack into drums, seal and store prior
to collection by specialized chemical disposal
company.
Catalyst Absorb into vermiculite, remove to
landfill or controlled incineration. Wash down
remaining traces with copious water.
Accelerator Absorb into dry sand and dispose by
landfill or controlled incineration.
Release agents Wash down with water.
Mould cleaner Absorb into sand or earth, remove
to landfill or controlled incineration. Flush contaminated
area with water.
Fire hazards
Many resins and associated products are either
flammable or contain flammable additives. Styrene,
catalyst and acetone are particularly dangerous. Do
not smoke or use naked lights, oil burners or similar
heating devices in the working area. If a fire does
start, do not attempt to put it out with water unless
it is a catalyst. Dry powder extinguishers can be
used on accelerator, mould cleaner, acetone, resins
and release agents.
Fires can be started if catalysed but uncured
resins are thrown away. The waste resin will continue
to cure and the heat generated by the curing
process can ignite other waste materials. Therefore
unwanted resin should be left in a safe place until
it is fully cured; it can then be discarded without
risk of fire.
Questions
1 Explain the following abbreviations: RRIM, VARI,
PVAL.
2 State three advantages of reinforced composite
materials when used in vehicle body
construction.
3 List the main physical properties of glass fibre
composite materials.
4 Explain the advantages and disadvantages of
reinforced composite materials when used as an
alternative to low-carbon steel.
5 Explain briefly the function of (a) the releasing
agent (b) the gel coat (c) the catalyst.
6 Describe the process of contact moulding.
7 Explain the purpose of pre-accelerated resins.
8 With the aid of a sketch, show the lay-up of a
laminate in the mould.
9 Describe the type of tools that would be used to
trim the edges of reinforced laminates.
10 Describe the sequence of repair to a damaged
glass fibre composite body panel.
11 Describe how a patch mould can be used during
the repair to damage of a GRP laminate.
12 Describe the likely damage that would occur to a
panel made from GRP and which had been
subjected to a heavy blow.
13 Name the types of materials that could be used
to reinforce polyester resin.
14 Name three reinforcing materials that could be
added to a GRP moulding to give strength to the
laminate.
15 Explain why a GRP panel may offer better
resistance to minor damage when compared with
a low-carbon steel panel.
16 In the automobile industry, why is GRP limited in
use to a small specialist sector?
17 Describe, with the aid of a sketch, a test to show
that GRP is more elastic and less ductile than
aluminium sheet.
18 State the reasons why GRP has not replaced
low-carbon steel as the material used to
manufacture vehicle bodies.
19 State the reasons why certain manufacturers of
sports cars prefer to make their vehicle bodies
from GRP.
20 Explain how to repair a deep scratch in a GRP
body panel.
Automotive
finishing and
refinishing
17.1 History of automotive finishing
No repair to a vehicle is complete until it has been
painted to match the rest of the vehicle and is rendered
undetectable. This part of the operation is
carried out by the spray painter, who must have a
knowledge of the type of materials used in the
repair shop in order to help him select the best
process for refinishing the vehicle concerned.
Nowadays the spray painter has the help of the
paint manufacturers, who can supply him with literature
to cover every painting process, but his
predecessor, the coach painter, had to have a very
solid understanding of the materials at his disposal.
As the term coach painting implies, this is a craft
which dates back long before the days of the motor
car. Reference is made to coach painting in the
diary of Samuel Pepys, in which he makes mention
of the buying and repairing of a second-hand
horse-drawn carriage. The amount charged to him
for the repainting was sixty pounds which, four
hundred years ago, would be a fairly large sum of
money. The time involved for the repainting was
one month which, bearing in mind the type of paint
used, was not unreasonable.
In those days the painter not only mixed his own
colours but actually manufactured his own paints,
using a pestle and mortar to grind the pigment and
oil together. The choice of materials available was
rather limited, but it is to the credit of the craftsmen
of those days that the finished appearance was of a
very high standard and extremely durable. Perhaps
the best protective pigment available to the craftsman
was white lead, and he made full use of it. The
lead paste was mixed with linseed oil and applied
by brush, one coat every second day, being too slow
in drying to allow for more frequent coatings.
Several coats were applied, each being rubbed
down smooth prior to the application of the next
coat. When the work was judged to be ready for the
colour coats, these were also applied in several layers,
being too transparent to cover in one solid coat.
When the painting was completed, the sign
writer took over and embellished the coach with
line work and heraldic emblems. One of the most
widely used materials was silver leaf; gold leaf was
not then available. Following this part of the work,
as many as seven coats of varnish were applied.
The varnish, being rather yellow in colour, tended
to enhance the silver leaf by tinting it amber and
giving it the appearance of gold. This slow, laborious
and costly painting process continued almost
without alteration right up to the end of the nineteenth
century and the birth of the motor car. Paint
manufacturers had, however, come into being, with
a consequent improvement in the range and quality
of materials at the disposal of the painter.
The early motor car, like its predecessor the
coach or carriage, was of coach-built construction,
and the existing methods of painting were suitable
for this type of vehicle. However, as the motor car
increased in popularity and demand for it grew, a
new and faster method of production had to be
found. This was achieved by the advent of pressed
steel construction, but the paint process caused a
bottleneck to production and so research was carried
out to solve this problem.
The answer came with the development of cellulose
lacquer which, though not a complete answer,
was nevertheless an extremely fast drying material
which allowed for several coats to be applied in
one day (speed, of course, being the main criterion).
Being so rapid in drying, cellulose was not
suitable for brushing purposes and so the application
of paint using a spray gun came into its own.
By 1930 all new motor cars were being finished by
this method. The material, however, was lacking in
solid content, and consequently several coats had
to be applied to achieve a coating of worthwhile
thickness. Another time consuming factor was that
the finished vehicle had to be burnished to obtain a
high gloss.
Around 1935 cellulose-lacquer based paints
were combined with other synthetic materials to
produce a paint which dried in thirty minutes, had
better ‘build’ qualities and thus required fewer
coats. It also reduced the burnishing time and so
eased the bottleneck which still existed in the paint
section of the production line.
During the Second World War a great deal of
research was carried out, and success achieved, with
thermosetting paints which could be force dried at
elevated temperatures. These paints provided a hard
glossy finish, required fewer coats than the cellulose
materials and were more chemically resistant. A further
advantage was that the finish required no burnishing
or polishing. In all, this was a paint ideally
suited to the expanding motor vehicle industry, and
by the early 1950s all new motor cars were being
finished in these stoving synthetics.
As well as improvements in finishing materials,
changes in painting techniques were being evolved.
Perhaps the most revolutionary changes were introduced
in the application of the priming paints,
mainly in the field of dip application. In this method
the entire body shell is completely immersed in a
tank of priming paint (which is specially formulated
for this purpose), is withdrawn, allowed to drain,
and is then passed on to a stoving oven for baking.
Stoved synthetic finishes became the accepted
finish on new motor vehicles, but difficulties were
experienced in refinishing damaged areas as a
result of colour fading. Though the colours did not
fade drastically, they did, however, fade sufficiently
to give the refinisher a difficult job to
obtain a perfect match.
In 1963 Vauxhall introduced a finish on their new
Viva model which the paint manufacturers claimed
had better colour stability. This was the acrylic
resin stoving finish which was produced with the
cooperation of the plastics industry (being of the
thermoplastic type). By 1965 Ford had changed all
of their colours to a high-bake acrylic finish, which
was a product of the paint industry only, being
thermosetting. BLMC, Rootes, Standard, Triumph
and Rover followed suit by changing most of their
colours to the acrylic range. Acrylic paints, as well
as possessing good colour stability, are durable,
have good gloss and are easily polished.
The method by which the priming coat on modern
vehicle body shells is applied is known as
electrodeposition (Figure 17.1). A large dip tank
containing 2500 litres of a water-borne paint is
included in the production line. An overhead conveyor
carries the body shells from the pre-cleaning
area to the dip tank. The paint is charged with electricity
and the shell is earthed through the conveyor.
The thinner of the paint, being water, acts as
an electolyte; the paint solids, i.e. pigment and
Automotive finishing and refinishing 575
Figure 17.1Electrodeposition of priming coat
576Repair of Vehicle Bodies
binder, are ionized and are attracted to the earthed
car body. An even coating of paint is thus applied,
even on thin metal edges. The thickness of the
coating can be varied according to the electrical
potential introduced. When the car body moves out
of the tank, surplus paint drains out of it and the
shell is then rinsed off under sprinklers, which
does not affect the electrodeposited coating. The
car body is then dried off and baked.
As to the future of motor vehicle finishing, it
seems reasonable to expect water-borne paints to be
developed to such an extent that they will become
the accepted finish on new motor vehicles. Looking
even further ahead, it could be that the car body will
be formed entirely of a moulded plastic which could
be self-coloured. Should this come about, damaged
areas could be removed and replaced with a new
section which is already coloured to match the rest
of the car. However, there will still be a place for the
refinisher, as car owners will, in all probability,
desire the occasional colour change on their car. In
all, there have been many developments since the
days of the coach painter and his homemade paints.
17.2 Glossary of terms used in spray
painting
In order to be able to appreciate more fully the
descriptions of processes and practices in the
paint shop, the reader should make himself
acquainted with the following trade terms and
items of equipment.
Air delivery The actual volume of compressed air
delivered by the compressor after making allowances
for losses due to friction. It is measured in
litres per second.
Air duster A tool which, when fitted to an air
line, is useful for blowing water from recesses and
for drying a surface quickly prior to painting.
Air pressure The pressure of air which has been
mechanically compressed. It is measured in bars or
pounds per square inch (psi).
Air receiver A reservoir or storage tank to contain
compressed air.
Atomization The breaking up of paint or other
materials into very fine particles. Good atomization
is essential in spray painting.
Double-header coating This results from the
practice of spraying one coat immediately after
another without allowing a flash-off period.
Dry coating Several thin coats of paint can be
applied fairly rapidly if they are sprayed ‘dry’.
This is done by increasing the ratio of atomizing
air to paint at the spray head of the gun. Dry coating
is particularly useful when carrying out local
repairs to paintwork.
Feather edging The rubbing down of a damaged
area of paintwork until there is no perceptible edge
between the paint and the substrate.
Feathering the gun To ease the pressure on the
gun trigger whilst spraying, thereby reducing the
volume of paint passing through the fluid tip. This
is done mainly when spraying local areas to
achieve a feather edge.
Flash off To allow the greater part of the more
volatile solvents in a sprayed coat of lacquer or
enamel to evaporate before proceeding with the
application of another coat or with stoving.
Fluid cup A container for the paint attached to
the spray gun in conventional spray painting, or a
separate item in pressure feed systems connected
to the spray gun by means of a fluid hose.
Fluid nozzle The orifice in the fluid tip.
Ground coats The paint coats between the primer
and finishing coats. Ground coats are usually of a
similar colour to the enamel.
Guide coat A thin coating applied as evenly as possible
over a surface to be rubbed down. Following
rubbing down, no trace of the guide coat should
remain, so that complete flattening of the surface is
achieved. The guide coat should obviously be of a
contrasting colour to that over which it is applied.
Hold-out The degree of imperviousness of a
dried paint film. Some filler coatings in particular
are porous and so they tend to absorb the binder or
medium of finishing coats, thus reducing their
effectiveness as a glossy finish.
Matt finish A surface finish which has no glossy
effect.
Shrinkage This refers to the manner in which
some paints decrease in size not only vertically but
also horizontally. As with sinkage, this is caused
by solvent evaporation. Nitrocellulose materials
are particularly prone to this phenomenon, which
can affect the adhesion to the substrate.
Sinkage This trade term can have two interpretations.
When paint is applied over a particularly
porous surface it will sink into it, and if this paint
is a finishing material the gloss will be impaired.
The other explanation of the term brings in solvent
Automotive finishing and refinishing 577
evaporation. The solvents or thinner added to
paints to reduce the viscosity evaporate during the
drying process and consequently some of the liquid
content of the paint vanishes. When this happens
the paint film becomes thinner and projections on
the substrate come through it.
Tacking off To wipe over a surface with a specially
treated cloth, which is slightly sticky, to
remove dust. Tacking off is essential before applying
finishing coats.
Viscosity The degree of resistance to flow of a
liquid. More simply, it refers to the thickness or
otherwise of a fluid such as paint.
17.3 Basic composition of paint
Pigments Fine solid particles which do not dissolve
in the binder. They give colour and/or body to
the paint. Some pigments possess good anti-corrosive
properties and are used in paints designed to give protection
to the substrate. Extenders are cheaper than
pigment, but when used in the correct proportions
they carry out many useful functions such as
improvement of adhesion and ease of sanding.
Binder Reacts to form a film, and binds the pigments
together and to the surface. The binder is
often referred to as the medium of the paint.
Thinner Some of the liquid of the paint is often
withheld from the paint container and supplied
separately as a thinner. The user adds thinners to
adjust the viscosity to suit his requirements.
Additives Small quantities of substances which
are added to carry out special jobs. Wax in varnish
creates a matt finish, and silicones in metallic paint
give a hammer finish.
Figure 17.2 illustrates the composition of paint.
17.4 Types of paint
Cellulose synthetic
This dries by the evaporation of the solvent. The
main advantage of this material is rapid air drying.
However, there are a number of disadvantages.
The coating dries rapidly only when thin
films are applied, otherwise drying is delayed by
solvent retention. The high proportion of solvent
used (60 per cent in most cases) results in shrinkage
which causes the film to adhere poorly to the
substrate. The absence of chemical change means
that the dried film does not increase in chemical
resistance, and is readily softened by the original
solvent.
Oil paints
The drying of an oil paint depends on the ability of
certain drying oils to dry by a reaction that
involves atmospheric oxygen, a process which is
confined to relatively thin films.
Synthetic paints
These are mixtures of drying oils and synthetic
resins. The most obvious limitation of a paint based
solely on a drying oil is slow drying. To improve
this property and to give tougher films and improve
the gloss, a resin is added to the oil and they are
cooked together for a period so that they chemically
combine. The varnishes produced can be divided
into two main classes based on their oil to resin
content: long oil and short oil.
Stoving paints
These are also mixtures of oils and resins that
require exposure to an elevated temperature to
Figure 17.2Composition of paint
578Repair of Vehicle Bodies
produce a cure (dried film). The time of exposure is
mainly dependent on the temperature: 60 minutes at
a temperature of not less than 138 °C; and 10 minutes
at a temperature of not less than 205 °C.
Blacking paints
Chassis black is a cheap black paint generally
based on bitumen. It has good adhesive qualities
on bare metal and is a good rust inhibitor.
Tyre black is also a cheap black paint, being
of low viscosity. Several proprietary brands are
available.
Two-pack paints
These are probably the most widely used paints in
the vehicle refinishing trade, with more than 80 per
cent of refinishers preferring them. They present
special health hazards, and the user should be
equipped with an air-fed mask and face visor to
prevent inhalation of the vapours when spraying
(Figure 17.3). A canister mask of the CC type can
be used as an alternative, but these can prove to be
expensive as the canister is only useful for 15 minutes
continuous use and should then be discarded.
Precautions should also be taken to prevent the
mixed material or spray vapours making contact
with the skin, as this can cause dermatitis.
These paints consist of a base material and a
catalyst or activator. When they are mixed together,
a chemical reaction takes place which results in
complete polymerization. Two-pack (or 2K) paints
have a limited pot life after mixing, but when curing
is complete they can equal stoving paints in
hardness and durability. They are characterized by
high solids content and low solvent content, which
results in high build and good scratch filling with
the minimum number of coats, thus resulting in
savings on labour time and overspray wastage.
The gloss from the gun is good and no burnishing
or polishing is needed unless dirt is present in the
finish. Should this be the case, the finish can be wet
flatted with P1200 paper, using soap as a lubricant,
and burnishing can be carried out using fine rubbing
compound and a polishing machine of 6000
rev/min. A clean, dry lambswool mop is recommended
for best results.
Materials included in this group are: acrylic and
polyurethane primer undercoats and finishes,
including base-coat-and-clear finishes; epoxy resin
primers and finishes; and polyester spraying fillers.
Figure 17.3DeVilbiss Pulsafe breathing air kit showing half-mask and visor outfits (DeVilbiss Automotive
Refinishing Products)
Automotive finishing and refinishing 579
It is common practice in most refinishing paint
shops to force dry these materials using low-bake
ovens on large areas for 30 minutes of 60 °C, or
using infrared lamps on small areas.
Low-bake finishes
These are modified stoving paints which can be
completely cured at a temperature between 66 and
93 °C. The material was formulated for the refinisher
to enable him to match more closely the original
finish of the car manufacturers. These are
now losing favour to two-pack materials.
17.5 Materials used in refinishing