Liquid
Following the removal of the paint by either of the
methods described, the bare metal should be treated
with a proprietary phosphating liquid. These liquids
(there are several brands available) have an acid
content, mainly phosphoric, which etches the metal
and completes the derusting of it. In addition to
etching the metal, phosphating solutions deposit a
layer of iron or zinc phosphates on to the metal
which inhibits corrosion. These liquids should be
used according to the manufacturer’s instructions,
but are generally diluted in the ratio 1:1 with water,
applied with an old paintbrush and washed off with
water after about fifteen minutes. The liquid, being
of acid content, should not be allowed to come into
contact with the skin, eyes or clothing of the operator,
and rubber gloves should be worn to protect
the hands. In the event of its accidentally doing so,
the affected part should be rinsed thoroughly in
running water. Following drying of the bare metal
with an air duster and wash leather, the area is now
ready for repainting.
17.14 Finishing and refinishing processes
There are four main car refinishing paints at the
refinisher’s disposal: cellulose synthetic (half-hour
enamel) paint; acrylic resin paint; low-bake synthetic
paint; and two-pack paints. Cellulose synthetic
and low-bake synthetics are used as repair
materials over the high-bake synthetics applied by
the car manufacturers; in addition, of course, they
can be used for complete resprays. Acrylics are
best used on repair work over an original highbake
acrylic finish, but can also be used as a refinishing
material on complete resprays. To determine
whether a paint is suitable for repairing a particular
job, a flat area should be chosen and a wet coat of
the paint sprayed on to it. If wrinkling or lifting
occurs this is proof that the solvent is too strong
and will probably lift the existing finish.
The following paint systems are typical of those
carried out in refinishing shops. They do not follow
any one particular paint manufacturer’s specifications
but are intended as a guide to the use of the
various paint types. When using a particular brand
of paint, the operator should always follow the
maker’s instructions as to viscosities, drying times
and temperatures. The processes outlined are based
on the assumption that the vehicle has been prepared
for refinishing as described in Section 17.13,
followed by the necessary masking up.
Coach finish
This is the traditional material, used in coach
painting workshops for many years, but it has lost
favour to more modern materials mainly because
of its lengthy drying time. However, in paint shops
where spraying equipment is not available, this
material still has a use. It is essential that the workshop
should be kept clean, otherwise dirt in the finish
is a certainty owing to the prolonged period of
paint film wetness.
The finishing material can be sprayed either
hot or cold and can also be brushed. It is suitable
for application on all vehicle construction materials
including wood. Undercoats may also be
sprayed hot when thinned 6:1 with white spirit.
The paint system is of the simplest type. For
example, for metal surfaces the procedure is as
follows:
1 Prepare the surface.
2 Carry out any necessary masking.
3 Wipe the surface down with a proprietary spirit
wipe or a mixture of methylated spirit and
water.
4 Apply one coat of self-etching primer, either
spraying or brushing type. Allow to dry and
then apply ground coats as soon as possible.
5 Apply two coats of ground coat:
(a) If cold sprayed, allow 2 hours between
coats.
(b) If hot sprayed, heat to 60 °C and allow
1 hour between coats.
(c) When brushed, allow to dry for 6 hours
between coats.
6 Should stopping up be necessary, this should be
done after the first ground coat which should,
of course, be allowed to through-dry completely.
A synthetic resin stopper should be
used and, when hard-dry, wet flatted with P360
wet-or-dry paper.
7 Should sanding down of the ground coats be
necessary, allow to dry for at least 6 hours, and
dry-sand using P400 lubricoat paper. Dust off
and tack off. If sanding down is not necessary,
the finishing material can be applied following
the flash-off times listed in point 5.
Automotive finishing and refinishing 603
8 Apply finishing enamel:
(a) When brushed, no thinning is required.
(b) When not sprayed, heat to 60 °C without
thinner.
(c) When cold sprayed, thin 6:1 with white
spirit or preferably with a thinner supplied
by the paint manufacturer.
Apply two coats as follows:
(a) Brushed: allow at least 6 hours between
coats, the time depending on workshop
temperature.
(b) Hot sprayed: one light coat; allow 30 minutes
and apply a double-header coating.
(c) Cold sprayed: one light coat; allow 60
to 90 minutes before applying a doubleheader
coat, but take care not to apply
too wet a coating otherwise runs will
result.
Spraying pressures should be in the order of
3–4 bars (45–60 psi). Spray gun fluid tips
should be of the smaller types (say 1.25 mm
diameter) to avoid flooding the job. Spray gun
distance from job is best about 200 mm.
9 Remove masking whilst the paint is at the
tacky stage. This will allow the edges to settle
down into place.
10 After overnight hardening, transfers, lining
and lettering may be applied.
