Primer
The priming coat is the first coat of paint on any
surface. Its functions are to gain maximum adhesion
to the substrate, to provide a sound base for
subsequent coatings, and on metals to act as a corrosion
inhibitor. The priming paint should be
selected to suit the type of surface on to which it is
to be applied.
Etch primer
The best types of etch primer are two-pack materials,
the base and activator being supplied separately.
They are mixed ten minutes before use, but
have a limited pot life (about six hours), although
longlife etch primers are now available. Brushing
activators are available for those shops where spray
painting is not practical. Etch primers have a fairly
high water absorption characteristic and should be
coated with surfacer or filler after the appropriate
drying time, to avoid moisture absorption from the
surrounding atmosphere. Special thinners are provided
for etch primers and should be kept for this
purpose only.
The pigment for this type of paint is zinc chromate
which makes it an ideal primer for aluminium,
although it can be used also with good
effect on most other metals. The activator contains
phosphoric acid which etches the surface, thereby
ensuring good adhesion. An extremely thin coating
gives best results. Once the base and activator have
been mixed together they should on no account be
returned to the tin of base.
Primer surfaces
A primer surfacer does the work of both a primer
and a surfacer.
Self-etching primer surfacers are available which
eliminate the need for using an etching primer
and then coating over with surfacer or filler. Selfetching
primer surfacers have gained in popularity,
the main advantage being improved adhesion to
metal substrates when compared with standard
primer surfacers.
Primer filler
A primer filler is similar in function to a primer
and a filler.
Polyester primer filler is a two-pack paint, first
used extensively on the European continent. When
introduced into Britain it was viewed with a certain
amount of distrust, as the claims made for it
(primer, stopper, surfacer rolled into one) appeared
too good to be true. There was good cause for this
distrust, as one of the difficulties encountered was
that blistering occurred when it was used with nitro
cellulose materials. Another factor against it was
that it required four hours to cure. However, modifications
have been made to it which have resolved
the problem of blistering and reduced the curing
time to two hours at an ambient temperature of
25 °C, which can be still further reduced to fifteen
minutes when force dried.
This paint is particularly suitable for use on
rough surfaces where heavy coatings are required
to level up the surface with the minimum of effort.
Several coatings can be applied wet-on-wet as it
dries by catalyst action. The amount of sinkage
after drying is virtually nil. Polyester primer/filler
possesses exceptionally good build qualities, and
when dry can be very easily smoothed down with
abrasive paper used either wet or dry. Face masks
should be worn when dry rubbing is carried out.
Best results are obtained if the paint is applied
with a gravity-fed spray gun. When spraying local
repairs the paint can be applied at the supplied viscosity,
but for a large area a small quantity of the
appropriate thinner may be added to obtain better
atomization and to provide a smoother coating.
Spray guns must be thoroughly cleaned immediately
after being used with polyester primer filler.
As the paint solvent is extremely volatile, the cap
must be screwed down tightly on the tin when not
in use. Polyester primer filler can be used with
580 Repair of Vehicle Bodies
most car refinishing processes and is very useful
when used with low-bake enamels.
Fillers and levellers
Surfacer
A surfacer is applied over the primer. Its function
is to build up the coating thickness, whilst filling
up minor defects such as scratches.
Filler
Filler is a heavy-bodied material used for levelling
defects which are too deep to be filled economically
with surfacer. Fillers are manufactured to suit the
method of application: there are spraying, brushing
and knifing types. Fillers are available as both singlepack
or two-pack, cellulose and synthetic types.
Stopper
Stopper is a putty-like substance used for filling up
defects too deep for satisfactory levelling with
either filler or surfacer.
Though deep indentations are normally filled up
by the body repair worker, the painter is sometimes
required to carry out levelling work. A two-pack
stopper, usually based on polyester resin and a
catalyst, is used for this purpose. It dries rapidly in
heavy layers, unlike cellulose or oil-based stoppers
which must be applied in thin layers with a drying
period between applications. Polyester stopper is
intended for use on bare metal or over high-baked
primers. It cannot be used as an intercoat stopper
over etching primers, or between enamel coats. It
can, however, be coated over with most standard
paint systems used in refinishing, including cellulose
synthetic, coach finish and low-bake synthetics.
The normal curing time of a polyester stopper
is about thirty minutes at 20 °C.
Sealers
There are three types of sealers used by the refinisher:
standard, isolators and bleed inhibitors.
Standard sealers have a low pigment and high
binder content. They are supplied ready for use and
are applied over the final coat of surfacer or spray
filler to provide hold-out of the finishing material
and promote higher gloss. They also reduce the
risk of crazing when applying acrylic lacquer-type
finish over cellulose-based undercoats.
Isolating sealers are more heavily pigmented and
are used to avoid a reaction between different types
of paint systems, e.g. when applying a lacquer-type
paint over a synthetic enamel.
Bleed-inhibiting sealers contain carbon black
pigment which is able to absorb floating colour.
These sealers are recommended when carrying out
a colour change over a colour which is suspected
of being a bleeder, i.e. some of the pigment or
dyestuff will float into the new coating and discolour
it. Several reds, particularly those containing
organic pigments, are prone to this behaviour.
Whichever sealer is used, it can only do its job if
it is a continuous film. For this reason it must not
be flatted down, through a light denib, carefully
done, is permissible.
Finish
Finish is the term used to describe the finishing
colour coats. They have a comparatively low pigment
content as opposed to surfacers and fillers.
The high percentage of the vehicle or binder provides
the glossy effect.
Abrasive papers
The type of abrasive paper mainly used by the
spray painter is known as wet-or-dry paper. The
abrasives used are silicon carbide and aluminium
oxide, and these are attached to a treated paper
backing by means of waterproof resin glue.
Wet-or-dry paper is available in various grades
ranging from 80D (coarse) to P1200 (very fine):
low numbers denote coarse grades and high numbers
identify the finer grades. This type of abrasive
paper is normally used in conjunction with water
to avoid a build-up of paint particles which would
affect the abrading effectiveness of the paper.
In addition to wet-or-dry paper, the spray painter
should have in his stock emery paper and production
papers. These are much coarser than wet-or-dry
papers and are used for rougher work such as removing
rust or mill scale. These two papers are most
effective when used with power tools in the form of
circular discs, being attached to the pad with specially
formulated disc adhesives. As they are normally
used dry, the particles are attached to the paper
or cloth backing with a good quality hide glue.
Abrasive papers fall into two categories, based
on the amount of space between the particles. If
Automotive finishing and refinishing 581
these are widely spaced the paper is referred to as
open coated and is used to remove paint or rust
which tend to fill up the spaces on finer grades.
When the particles are tightly packed the paper
is known as close coated and is used to rub down
smoother surfaces.
Masking
Masking tape
This is a paper tape, one side of which is coated
with an adhesive of a non-drying composition. It is
supplied in rolls in a variety of widths, but the most
widely used are those measuring 20 mm and 25 mm.
Wider tapes are considered to be uneconomical,
though narrower tapes such as 13 mm do have limited
uses. Where two-tone work is carried out, a
finer edge can be achieved by the use of gummed
paper than is possible with masking tape, but the
time spent on removing it makes it less popular.
Masking machines are also available in which a
roll of masking paper and various widths of masking
tape are mounted. As the paper is pulled out of
the machine, a strip of the tape is automatically
attached to one of the edges (Figure 17.4).
mind that newspaper is extremely porous and subject
to solvent penetration, and it is therefore recommended
that at least two layers be used over
existing paintwork. It is common practice in some
shops to oil or grease the newspaper, but this cannot
be recommended because of the risk of grease
contamination to those areas to be sprayed.
Burnishing
Burnishing compound
There are both burnishing pastes and polishing
liquids. These are generally emulsions of mineral
oils and water with the addition of an emulsifying
agent. They also contain mild abrasives to ‘cut down’
the final coat of enamel and promote a good lustre.
Burnishing compounds are also used during the carrying
out of local paint repairs to remove overspray.
Standard compounds and creams or liquids
contain ammonia to keep them fresh in the tin, but
ammonia can cause staining of clear coatings.
Special ammonia-free compounds and liquids are
available for the burnishing of clear-over-base
finishes.
Mutton cloth
In days gone by, butchers and slaughterers wrapped
joints of meat in this material – hence its name.
Mutton cloth is available in different grades: coarse,
medium and fine. The coarser grades are used with
rubbing compound to remove overspray when carrying
out localized paint repairs. The fine grades
are used with fine compounds and creams for final
burnishing to promote a deep gloss.
Solvents
The paint shop should be well stocked with the
appropriate solvents for the types of paint to be used.
The importance of using the correct thinner for a
Figure 17.4 Masking machine
Figure 17.5 Method of fixing masking tape to paper
Masking paper
Brown paper (kraft paper) is an ideal masking
material, though newspaper is very widely used for
this purpose (Figure 17.5). It should be borne in
582 Repair of Vehicle Bodies
particular type or make of paint cannot be overstressed,
as many painting defects can be traced to
the incorrect use of solvents. Solvents can prove to
be an expensive item, and in consequence a cheaper
form of cleaning solvent should be stocked for the
purposes of cleaning spray guns and equipment.
Solvents fall into two categories, high boilers and
low boilers. Those which require a high temperature
to bring them up to their boiling point tend to be
slow in evaporating and consequently slow in drying.
Low boilers, on the other hand, evaporate and
dry quickly. Low boilers are best used with primers,
surfacers and fillers where it is required to build up
the coating thickness fairly rapidly. High boilers can
be used in finishing coats to promote better flow,
helping to eliminate an orange-peel defect. A good
quality solvent, however, will contain both high and
low boilers in well balanced proportions.
17.6 Spray painting equipment
The items of equipment in a spray painting shop
are basically as follows: air compressing unit, air
line, air filter, pressure regulator; air hose and
finally spray gun.
Air compressor
When selecting an air compressor for a particular
workshop, one should calculate the volume of
compressed air that will be required to operate the
various tools throughout the workshop, such as
rubbing-down tools and spray guns. The size of the
compressor chosen should be capable of giving a
higher free air delivery than is required.
In a workshop where more than one activity is
carried out, such as panel beating and spray painting,
it is advisable to install a stationary two-stage
compressor. This should be bolted to the floor at
least 300 mm from any wall, and sited where it can
receive an ample supply of clean dry air. A twostage
compressor consists of an electric or fuel oil
motor, the compression unit itself, and a storage
tank. In addition to these basic components there
can also be various refinements in the way of
safety devices and operating switches.
Taking a piston-type compressor as an example,
the operational sequence is as follows. The motor
drives the compressing pistons, which are situated
within two cylinders set above the storage tank. Air
at normal atmospheric pressure is drawn into the
first cylinder via the air intake, to which is attached
a filter. The air is then compressed to an intermediate
pressure by the action of the piston moving
upwards and reducing the volume of the cylinder
(Figure 17.6). When a sufficient pressure has been
attained, the air is pumped through a chamber (or
intercooler) into the second smaller cylinder,
where it is compressed even further by piston
action and finally discharged into the storage tank
for subsequent use (Figure 17.7).
Figure 17.6 Single-acting two-stage compressor
Figure 17.7 Principal parts of a stationary two-stage
compressor
Automotive finishing and refinishing 583
The compressed air is stored in the receiver (or
storage tank) for a cooling-off period, during which
moisture and vapours within the air will condense
and collect on the floor of the tank. The very act of
compressing air will generate heat, and so the cooling
period is essential. In a further effort to keep the
air cool, the compression cylinders have external
fins to provide a greater surface area for the heat
generated within them to dissipate into the atmosphere.
In addition to storing the compressed air and
providing a cooling-off period, the air receiver acts
as a buffer between the compressor and the spray
gun, blanketing out pulsations so the compressed
air can be drawn from it at a steady even pressure.
It is important that the size of the air receiver be
in direct proportion to the size of the compressor.
Contrary to popular belief, air volume is more
important to the spray painter than is air pressure.
A typical suction-fed spray gun will require 3–4.5
litres of free air delivered (FAD) per second to
allow it to operate satisfactorily. The gun must
have this volume of air from the air receiver
regardless of the pressure. In a typical two-stage
piston-type compressor, the free air delivered will
only be 70 to 75 per cent volumetric efficiency.
Free air delivery is in consequence the prime factor
to consider when purchasing a compressor.
Air lines
The compressed air is drawn from the air receiver
and led to the air transformer through a galvanized
tube known as an air line. The three main requirements
of an air line installation are:
1 Low pressure drop between the compressor
plant and the points of air consumption
2 Minimum of air leakage
3 High degree of contamination filtering throughout
the system.
Pressure drop in an air line is caused by the frictional
action of the air molecules on the inside surface
of the pipe, as well as pressure build-up at
angles. The greater the distance between the compressor
and consumption points, the greater will be
the pressure drop. For this reason the air line must
be of sufficient internal diameter to carry a satisfactory
volume of compressed air into the spray
room. For the average refinishing shop an internal
diameter of 25 mm will suffice. Pressure drops can
be increased where angles are introduced into the
system, and in consequence the number of angles
should be kept to a minimum. Also the more couplings
there are in the installation, the greater is the
risk of air leakage with a consequential drop in
pressure.
It is inevitable that some moisture vapour will be
drawn into the air line from the air receiver, and for
this reason the air line should slope down towards
the air receiver to enable condensed moisture to
run into it rather than to the air transformer. The
service line leading to the transformer should be
tapped from the top of the main air line, and
should preferably be in the form of a U bend rather
than elbow joints with right-angled bends on them
(Figure 17.8).
Figure 17.8 Air line installation
Air transformer
This consists of two units: a condenser or filter,
and a pressure regulator. The condenser allows the
compressed air to expand into a chamber, thus
assisting cooling, and then removes moisture from
it by means of a removable filter. A drain cock is
situated at the base of the chamber to drain off
accumulated impurities periodically. The regulator
is a reducing valve with which to reduce the air
pressure from the compressor to that required for
spraying. The air transformer should be fitted with
a pressure gauge giving an accurate reading of the
pressure of air passing through the regulating valve
(Figure 17.9). A small lightweight filter can be fitted
to the handle of the spray gun as an additional
safeguard in conditions where exceptional humidity
exists.
584 Repair of Vehicle Bodies
Air hose
The compressed air is led from the transformer to
the spray gun by means of an air hose. This consists
of a rubber tube covered with cotton braid
enclosed within a rubber covering; the three layers
are vulcanized into one (Figure 17.10). Multibraid
hoses are available for high-pressure work. At each
end of the hose are couplings for attachment to the
transformer and the spray gun.
using an air hose of diameter 8 mm over the same
length and at the same supplied pressure, the drop
would only be 0.2 bar (3.5 psi). From this it can be
seen that a hose of 8 mm diameter will give best
results on lengths exceeding 4 m. When working in
a spray booth, the spray painter rarely, if ever,
requires an air hose greater than 6 m in length.
17.7 Types of spray gun
Of all the tools and techniques used in paint shops,
the spray gun and spray painting have provided the
most satisfactory method of applying paint. Unless
there is a complete change in the design and construction
of the motor vehicle, the spray gun will be used
for many years to come. The spray gun, like all tools,
is only effective in the hands of a skilled operator, and
therefore a painter should know as much as possible
about what has become the main tool of the trade.
A spray gun is a precision instrument which uses
compressed air to atomize the fluid paint and break
it up into small particles. The air and paint enter
the gun through separate passages, mix, and are
then ejected at the front of the gun.
Spray guns can be divided into groups:
1 By methods of paint supply, such as suction
feed, gravity feed or pressure feed
2 Those with detached or attached paint containers
3 Internal mix or external mix types
4 Bleeder or non-bleeder types.
The most widely used spray gun is the suctionfeed,
external-mix, non-bleeder model.
Paint supply methods
Suction-feed gun
With this type, a stream of compressed air creates
a vacuum at the fluid tip which allows atmospheric
pressure within the fluid cup to force the
paint up the fluid tube to the fluid tip and air cap.
The paint container (fluid cup) is limited to one
litre (1000 cm3) capacity to enable the gun to be
handled without fatigue. The suction-feed spray
gun is easily identified, as the fluid tip protrudes
slightly beyond the air cap (Figure 17.11).
Gravity-feed gun
The fluid cup is mounted above the spray head and
paint is fed to the gun by the force of gravity. The
fluid cup is usually limited to 0.5 litre capacity,
Figure 17.10 Single-braid air hose
Figure 17.9 DeVilbiss DVFR-1 filter regulator
assembly and DVF2-2 filter regulator coalescer
(DeVilbiss Automotive Refinishing Products)
Though the interior wall of the air hose is
smooth, it will still create a certain amount of
resistance to the flow of air, particularly when long
lengths are used. Hoses with an interior diameter
of 6 mm should never be used in lengths exceeding
4 m because of the high pressure drop encountered.
For example, if the air transformer is delivering
air at a pressure of 3.5 bar (50 psi) the pressure
drop over 4 m would be 0.7 bar (10 psi). However,
Automotive finishing and refinishing 585
which makes the gravity-feed gun unsuitable for
the spraying of large areas. However, it is very useful
for painting local repairs where heavy-bodied
fillers are applied, and where rapid colour changes
are necessary (Figure 17.12).
Pressure-feed gun
This type of gun sprays paint that has been forced
from the paint container by compressed air. The air
cap of this gun is not designed to create a vacuum,
and the fluid tip is flush with the front of the air
cap (Figure 17.13). Pressure-feed guns are used
where a large quantity of a particular paint is to be
sprayed, or where the material is too viscose or
Figure 17.11 Typical suction-feed spray gun
Figure 17.12 Gravity-feed spray gun (DeVilbiss
Automotive Refinishing Products)
Figure 17.13 Pressure-feed spray head
1 Air cap
2 Fluid tip
3 Fluid needle
4 Fluid control screw
5 Spreader control
6 Air valve
7 Trigger
8 Fluid packing nut
9 Gun body
10 Air inlet
11 Fluid inlet
586 Repair of Vehicle Bodies
heavy to be siphoned from the fluid container as in
suction-feed guns.
The fluid container or pressure vessel is connected
to the gun by means of a reinforced fluid
hose, and normally ranges in size from 2 to 25
litres. The smaller pressure vessels can be carried
in the operator’s free hand (Figure 17.14), but ones
from 10 litres capacity upward can be mounted on
wheels for easy portability (Figure 17.15).
Figure 17.14 Remote cup with gun (2.3 litre
capacity) (DeVilbiss Automotive Refinishing Products)
Figure 17.16 Internal-mix spray head
Figure 17.17 External-mix suction-feed spray head
Figure 17.15 Typical pressure-feed tank
Bleed methods
Bleeder gun
This is designed without an air valve. Air continually
passes through the gun, thus preventing a buildup
of pressure in the air hose. The gun is usually
used with small compressors having a limited output
and having no unload or pressure switch. The trigger
on the gun controls the flow of paint only.
Mix methods
Internal-mix gun
This gun mixes air and paint inside the air cap, and
is used with low air pressure to apply show-drying
materials (Figure 17.16).
External-mix gun
This is the most widely used type of gun, and
can be used to spray most types of paint. It is the
best type of gun for spraying quick-drying materials.
The air and paint mix beyond the air
cap, and perfect atomization can be achieved
(Figure 17.17).
Automotive finishing and refinishing 587
Non-bleeder gun
These are equipped with an air valve. The trigger
controls both the flow of compressed air passing
through the gun and the flow of paint.
