Quick air drying synthetics
This type of material offers special advantages for
low-cost resprays and rapid finishing of commercial
vehicles. The main advantage is that it is suitable for
application over all types of existing finishes, and is
suitable for hot or cold spray application.
The rapid respray system is as follows:
1 Clean off all traces of traffic dirt, grease, wax
and silicone polishes by using water miscible
cleaning solution.
2 Feather edge all damaged areas including parts
damaged by stone chips (generally on sills).
Treat bare metal with derusting liquid.
3 Spot prime any bare metal areas with cellulose-
based primer surfacer, and allow to dry
for half to one hour. Stopping, if necessary, is
best done before priming with two-pack polyester
stopper.
4 Wet flat the whole vehicle with P600 grade
paper, using liquid detergent or water miscible
solution as a lubricant and to remove wax.
Rinse, dry and tack off.
5 Thin the enamel to a viscosity of 23–26 seconds
(4:1). Spray one coat and allow to flash
off for 10 minutes, then apply a double-header
coating. The finish may also be applied hot, in
which case it requires no thinning. The paint is
heated to a temperature of 70–80 °C, and one
coat will be sufficient to give the required film
thickness. The flash-off times between coats
recommended by the paint manufacturer must
be strictly observed. If too great a time lapse is
allowed, then problems of lifting or wrinkling
may be encountered.
Low-bake enamels
These are modified high-bake enamels rather similar
in composition to those used by motor vehicle
manufacturers. Though developed primarily for the
refinishing trade, they are in fact used by vehicle
manufacturers in the rectification of damaged or
faulty finished vehicles. One of the problems
encountered by the refinisher has always been to
obtain a close match to the original finish. When
exposed to sunlight some colours have a tendency
to fade, whilst a certain amount of discoloration
takes place in others; some whites, for instance,
tend to yellow with time. In addition to this, some
pigments combine very well with a synthetic
medium but not so well with cellulose-based vehicles.
As the latter have always been the most widely
used materials in refinishing, colour matching thus
presented problems. These would have been less if
refinishing shops were equipped with equipment
similar to that of the vehicle makers, allowing them
to use the same finishing materials. This, of course,
would require a tremendous capital outlay which
would be well beyond the reach of the refinisher,
and would in all probability never be recovered.
In an effort to improve refinishing techniques, to
reduce drying times and to obtain faster production,
low-bake enamels (i.e. paints that cure at
lower temperatures than the original high-bake finish)
were developed. Being similar in structure to
the high-bake material, these paints offer a closer
match in colour and texture than do cellulose synthetic
materials. Burnishing and polishing is seldom
required as these paints provide a good gloss
from the gun.
Equipment
An obvious requirement for the drying of low-bake
enamels is a stoving room, and various types are
available. A stoving room can be acquired which can
be connected to an existing spray booth, or where
workshop space is limited a combined spray booth
and low-bake oven can be installed (Figure 17.52).
The obvious limitation of a combined spray booth
and low-bake oven is that no further paint spraying
can be done during the stoving schedule, in addition
to which valuable working time can be lost whilst
waiting for the booth to return to a comfortable
working temperature.
Where workshop space permits, a better proposition
would be to install a low-bake oven which can
be joined up to an existing spray booth with sliding
shutter doors to seal the two areas (Figure 17.53).
This can be further improved upon by having
entrance/exit doors at each end of the unit which
would provide a flow-line system of painting and
stoving (Figure 17.54), thus reducing time wastage
on vehicle movement. The type of installation must
obviously be governed by the size and shape of the
workshop (Figures 17.55–17.59). A further variation
Automotive finishing and refinishing 609
and have good adhesion properties when baked.
Following rubbing down and cleaning off, the surface
can be sprayed with synthetic resin primer
surfacer if it is of a rough nature. As previously
stated, these primers have exceptional filling properties,
but when used with low-bake finishes
the stoving schedule (1–1 hours at 82–93 °C) can
prove to be too long to be economical. Where the
surface is not too rough, a better method would be
to use cellulose primer surfacer thinned 1:1. Slight
imperfections should be levelled with cellulose
stopper, which must be over-coated with surfacer.
The surfacer can then be stoved for thirty minutes
at 70–80 °C. A third material finding favour is the
polyester primer filler described in Section 17.5.1,
which obviates the need for stopper on small
imperfections and provides a single coat build-up
to the finishing stage. As previously stated, several
coats can be applied wet on wet without flash-off
periods, and force dried for 15 minutes at about
75 °C. A further advantage is that polyester stopper
can be used, if necessary, between coats of polyester
primer filler. This material should be overcoated
with cellulose primer surfacer before
applying the finishing material. Whichever of the
above materials is chosen, the final coating must
be rubbed down wet, and following drying and
tacking off, the colour coats can then be applied.
Spraying
The finishing material is thinned 7:2 and sprayed
in one single coat followed by a double-header
coat, with a 15 minute flash-off between coats.
Another flash-off period of 15 minutes should be
allowed before stoving.
Preparation for stoving
It is current practice in the motor industry to repair
areas damaged during assembly with low-bake
enamels and then to stove the whole body fully
trimmed. The temperature inside the vehicle does
not rise sufficiently to damage the trim provided
that the doors and windows are properly closed.
Before stoving a vehicle refinished with low-bake
enamel, check that doors and windows are closed,
remove all exterior plastic fittings, increase tyre
pressure by 0.5 bar (5 psi), check that the petrol
tank is not too full and, if duotone work is being
done, remove any masking. It is not necessary to
remove the battery.
Figure 17.52Spray booth combined with low-bake
oven
is shown in Figure 17.60, in which the vehicle being
painted is moved sideways on rails and bogies.
Apart from the stoving room, the usual spray
painting equipment used in refinishing shops is
required.
Preparatory work
Deep indentations in the vehicle body can be filled
with polyester stoppers, which cure very quickly
610Repair of Vehicle Bodies
Stoving
The usual stoving schedule is 30–40 minutes at
80–100 °C, following which the body should be
allowed to cool for about 45 minutes (longer if
necessary) before further masking up for duotone
work is carried out. Certain parts of the car will be
sheltered from the heat, and consequently paint
applied to these parts will not be fully cured. A
converter liquid is available from most manufacturers
of low-bake paints which can be added to the
enamel for brushing in door edges and insides of
boot lids and bonnets, or these areas can be
Figure 17.53Low-bake oven joined to spray booth
Figure 17.54Oven joined to spray booth, with through doors for flow line
Figure 17.55Spraybake exterior units, height 3 m
(Spraybake Ltd )
Figure 17.56Multiple installation showing drive
through and paint mix room (integral) between units
(Spraybake Ltd )
Automotive finishing and refinishing 611
sprayed if the viscosity is reduced to 22 seconds
with the appropriate thinner. Any areas touched in
thus will dry quite hard at the normal paint shop
temperature.
Finishing
Polishing of low-bake enamels is not normally
necessary as a good gloss from the gun is obtainable,
but it should be borne in mind that these
paints remain more open, i.e. wetter, than more
coventional materials during the flash-off period
prior to stoving. For this reason, a clean workshop
is a necessity; otherwise dust may settle on the
Figure 17.57Multiple installation showing side
loading doors and track, and housing IRT arch
(Spraybake Ltd )
Figure 17.58Corner installation showing two
low-bake units, height 2.5 m (Spraybake Ltd )
Figure 17.59Interior shot of Green Booth designed
to meet EPA regulations (Spraybake Ltd )
Figure 17.60Joined oven and spray booth, with
sideways flow
612Repair of Vehicle Bodies
still wet film which will necessitate flatting and
burnishing to obtain a smooth finish. The nibs
should be removed with P800 grade wet or dry
paper and the flatted area brought up to a high
gloss with burnishing compound, followed with a
good-quality car polish. Low-bake finishing can
be carried out over both stoving and nitrocellulose
finishes providing they are in a sound
condition. Red or maroon cellulose finishes
which might bleed are best coated with a sealer
before commencing the spraying of the surfacer
coatings.
Developments
Apart from the cost of the stoving equipment, one of
the reasons why low-bake finishes were slow to find
popularity with refinishers was that it meant having
to increase the already extensive stock of paints normally
carried in refinishing shops. With this in
mind, paint manufacturers have developed a type of
thinner which, when added to their half-hour enamel
(nitrocellulose based), will convert it to a low-bake
cellulose enamel. Over 2000 car colours are available
in this material, and so colour matching does
not present too many problems. The cellulose stoving
enamel should be thinned with the converter
thinner to a viscosity of 23 to 26 seconds with a BS
B4 flow cup. Spray either three single coats or one
single and one double-header coat at 4–5 bars
(60–75 psi), allowing 15 minutes flash-off between
coats and another 15 minutes before stoving.
The stoving schedule recommended is 40 minutes
at 91–93 °C, during which the car body attains
a metal temperature of 80 °C for 30 minutes. This
temperature is not high enough to cause damage to
the car interior but is high enough to effect a complete
cure of the paint film.
Advantages
Low-bake refinishing, though costly to set up
initially, undoubtedly increases the potential productivity
of refinishing shops. Faster drying times
are obtained at almost every stage of the job, with
the result that vehicles are refinished much more
quickly than when using air-drying materials. This
leads to faster delivery of customers’ vehicles,
which is a very good selling point, and as the finished
article can be moved out of the working space
after cooling of the body, freer movement within
the workshop is obtained. Used properly, low-bake
finishes lead to a higher turnover of work, which in
turn increases the need for good organization
within the workshop (see Section 17.18).