Primers and dual-purpose primers

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

580Repair 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 17.4Masking machine

Figure 17.5Method 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

582Repair 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 17.6Single-acting two-stage compressor

Figure 17.7Principal 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 17.8Air 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.

584Repair 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 17.10Single-braid air hose

Figure 17.9DeVilbiss 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 17.11Typical suction-feed spray gun

Figure 17.12Gravity-feed spray gun (DeVilbiss

Automotive Refinishing Products)

Figure 17.13Pressure-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

586Repair 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 17.14Remote cup with gun (2.3 litre

capacity) (DeVilbiss Automotive Refinishing Products)

Figure 17.16Internal-mix spray head

Figure 17.17External-mix suction-feed spray head

Figure 17.15Typical 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.