Treatment with phosphating

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.