From pretreated bare metal
1 Spray one coat of acrylic primer filler thinned
to a viscosity of 21 to 23 seconds at 25 °C.
Allow 5 to 10 minutes to flash off.
2 Spray two coats of primer filler thinned 1:1, 26
to 29 seconds at 25 °C. Allow 5 to 10 minutes
between coats and 1 to 2 hours after second coat.
3 Apply cellulose stopper where necessary in
thin layers, allowing 15 to 20 minutes between
layers.
4 Wet flat stopper with 320–P400 grade paper,
dry off and tack off.
5 Spray stopper locally with primer surfacer, allow
5 to 10 minutes, then apply a full coat over the
entire surface. Leave to dry for 1 to 2 hours.
606Repair of Vehicle Bodies
6 Wet flat with P600 grade paper, rinse, dry off
and tack off.
7 Apply one wet coating of acrylic sealer, allow
to dry for 30 minutes, denib and dry. Do not wet
flat. The lacquer coats should be applied within
2 hours. Tack off before spraying the enamel.
8 Spray lacquer coats as necessary using one
light coat followed by a double header. Thin
the enamel 2:3 to a viscosity of 16 to 19 seconds
at 25 °C. Use the appropriate thinner only.
Should more than one coating of the lacquer be
considered necessary, allow a flash-off time of
5 to 10 minutes between coats. The lacquer is
touch dry after 15 minutes and can be safely
handled after 1 to 2 hours, depending on room
temperature. If necessary it can be burnished
and polished after overnight drying, though this
can be done after 4 to 6 hours if the enamel is
force dried. The air pressure used when applying
acrylic lacquer is between 3 bars (45 psi)
and 4 bars (60 psi) depending on the make of
spray gun employed.
17.14.7 Acrylic lacquer: complete respray
Over an existing finish
The method chosen here must obviously depend on
the condition of the paint film. If the surface
requires filling and stopping up, it should be wet
flatted with 280 grade paper using a water miscible
solution or liquid detergent. After rinsing and drying
off, the system described for bare metal can then be
used. If filling and stopping is not considered necessary
and the paint is sound, it can be wet flatted with
P600 grade paper and a solution of liquid detergent.
Following rinsing and drying off, tack off and apply
the sealer and lacquer coats as described.
17.14.8 Acrylic lacquer: local repair
Acrylic lacquer can be used to repair high- and
low-bake enamels, but is not recommended for
repairs to half-hour enamels or nitrocellulosebased
air drying finishes. Nor is it suitable for use
on wood or the repair of synthetic coach finishes.
A typical system for a local or spot repair is as
follows:
1 Degrease with a solution of liquid detergent.
2 Wet flat damage area with 180 grade paper
and feather edge.
3 Treat bare metal with metal conditioner or
phosphating liquid, rinse and dry off.
4 Spot prime with acrylic primer filler.
5 Stop up with cellulose stopper where required.
6 Wet flat with 280 grade paper and rubbingdown
block. Finish off with 320 grade paper,
rinse and dry off.
7 Spray in with acrylic primer filler sufficient
coats to level up the surface.
8 Wet flat with P600 grade paper, rinse, dry off
and tack off.
9 Spray over the repair with acrylic sealer.
10 Denib and dry. Wet flat around the edge of
repair with P800–P1000 grade paper. Burnish
surrounding panel.
11 Apply acrylic finish, thinned with quick-drying
repair thinner, in light coats. Finish off with a
double-header coat to obtain a smooth finish.
12 When dry, wet flat with P1200 grade paper,
burnish and polish.
Should the damage to the panel be too severe for
satisfactory or economical levelling up with cellulose
stopper, the two-pack polyester resin stopper
described in Section 17.5.2 could be used. This is
applied to the bare metal prior to the paint system.
It is best rubbed down and, after dusting off and
tacking off, coated with primer filler.
The use of a sealer coat at stage 9 may be eliminated
if the surface is carefully prepared. Sealers
are supplied ready for use, and it is difficult to
spray them without leaving an edge which is difficult
to remove. Providing that the original finish is
flatted 50–75 mm beyond the repair area, a satisfactory
job can be produced by spraying the finishing
coats on to the filler to slightly beyond its edge.
Care should be taken not to overlap the colour on
to unflatted enamel.
Metallic finishes
Practically without exception, metallic colours
being applied by car manufacturers in Britain are
based on acrylic resins. However, they present
problems not experienced when applying straight
colours, which are caused by the metallic particle
content. Without delving too deeply into the realms
of paint technology, a metallic finishing paint could
be described as a tinted, semi-transparent varnish
containing finely ground metallic particles such as
Automotive finishing and refinishing 607
aluminium, bronze and copper. Polished aluminium
flakes, because of their silvery metal appearance,
are the pigments most widely used. Because they
are lacking in opacity, these paints are generally
applied in several layers to achieve the desired
effect of colour depth and an even distribution of
the metal flakes. The best method is to apply a single
coat followed by wet on wet (double-header)
coats. The coverage may vary from colour to
colour. Apply as many coats as may be necessary.
Though car manufacturers favour acrylic-based
metallic paints, they are, however, available in cellulose
synthetic, slower drying types of spraying
synthetics, and, most widely used nowadays, twopack
synthetics. They are not suitable for brush
application. It is not necessary to outline a complete
paint system for metallic finishes; this section
deals with the application of the finishing coats.
The actual spraying of these has a great influence
on the finished appearance as regards colour. Spray
gun technique is very important. The gun should
be held at right angles to the surface and at a distance
of 150–200 mm approximately. If the distance
between the gun and the painted surface
varies, dark and light patches will result. A 50 per
cent overlap of gun stroke is essential to obtain an
overall even colour and texture.
When too wet a coating is applied, the metallic
flakes move freely within the wet film, and when
solvent evaporation takes place they are generally
in a fairly upright position. This tends to darken the
final effect, as light does not reflect too well from
the flakes in this position (Figure 17.51). Opacity
is also reduced and when sinkage takes place as a
result of solvent evaporation, the particles tend to
stick through the top skin of the paint film, causing
the finish to have a seedy appearance.
because the metallic flakes have a tendency to lie
parallel to the surface when a dry coating is applied.
When repairing a metallic finish, two options are
available to the refinisher. One is to extend the
repair to a natural break in the car body lines and
refinish the entire panel. Using this method there is
always the danger of ending up with a mismatch
when the masking is removed. A better method is to
apply the colour coats to the whole panel, remove
the masking and then recoat but extend the colour
further still, using a fade-out technique. Overspray
thus created should be overcoated with a clear
blend-in material to protect the overspray when burnishing
is done. When using this second method
with the base-coat-and-clear materials, it is best to
coat the panel with the clear coating first and, whilst
this is still wet, to apply the colour; this should then
be overcoated with further coats of clear.
There are too many variables involved to make it
possible to lay down a hard and fast system in
obtaining a perfect match on repair work. No matter
how much technical data is available, in the
final analysis it becomes a system of trial and
error, with the spray painter trying out various
spray gun set-ups, varying the air pressure, speed
of gun stroke, and distance from the surface. The
spraying viscosities vary according to the type of
material being used but a rough guide is as follows:
acrylic 16–19 seconds, cellulose synthetic
19–23 seconds, synthetic 23–28 seconds.
The damaged area should be levelled up as previously
described, and the whole of the surrounding
panel wet flatted with P400 grade paper. Following
drying and tacking off, the panel should be sprayed
with primer surfacer or filler and flatted with P600
grade paper. After drying and tacking, the colour
coats are applied. The function of this final coat of
surfacer is to equalize solvent penetration from the
finishing material. Surfacers and fillers are more
porous than enamels, and when the surrounding
panel is sprayed with the finishing colour only the
repaired area can be detected by a slight variation in
colour and texture. Where costs permit, a coat of
sealer could be applied prior to the finishing colour.
Light burnishing and polishing of metallic finishes
can be done after overnight drying in the case
of acrylic- and cellulose-based materials. With the
slower drying synthetics this process is best left for
about four days to allow for complete solvent evaporation
and to give the paint film time to harden
Figure 17.51Coatings too wet and too dry
Too dry an application will create a dusty effect,
too pale a colour when viewed head on, and too
deep a colour when viewed from the side. This is
608Repair of Vehicle Bodies
off. When using two-pack materials, burnishing
can be carried out after 16 hours at a workshop
temperature of 15–20 °C.