Cellulose synthetic (half-hour. 1 Rub down damaged area with P180 grade
enamel): local repair
1 Rub down damaged area with P180 grade
paper (wet) to obtain a feather edge. Wet flat
surrounding paintwork with P400–P600 grade
paper. Alternatively, dry sand with a dualacting
sanding tool using P80–P150 grade
paper.
2 Treat exposed metal with phosphating solution.
Automotive finishing and refinishing 605
3 Apply thin coat of etch primer to bare metal
only; alternatively, spray one coat of primer
surfacer directly on to bare metal, thinned 1:1.
4 Fill up defects with cellulose stopper in thin
layers.
5 Wet flat stopper with 320–360 grade paper.
6 Spray sufficient coats of primer surfacer or
filler (thinned 1:1) at 3 bars (45 psi) to bring
repair up to level of surrounding surface.
7 Wet flat with P600 grade paper.
8 Burnish surrounding area with rubbing compound
and a damp cloth to ensure a good
colour match and better blending in.
9 Tack off.
10 Spray colour coats, thinned 1:1 to a viscosity of
19–23 seconds, at a pressure of 3 bars (45 psi).
Spray several coats lightly until a good colour
match is achieved. Allow to dry hard.
11 Wet flat with P800 grade paper, dry off and
tack off.
12 Overspray the repair with a mixture of 75 per
cent thinner, 25 per cent colour, carrying the
spraying beyond the edge of the repair to
obtain a soft blend.
13 Allow to dry hard (preferably overnight) and
wet flat with P200 grade paper.
14 Burnish and polish.
Any masking off that may be required should be
done between stages 2 and 3. A coat of sealer may
be applied just prior to the colour coats to provide
better hold-out, thus obtaining a smoother finish.
Synthetic resin primers have not been included in
this process as they have a tendency to peel back
from the edges when rubbing down takes place.
Overspray from the spray gun can create unnecessary
work when carrying out local paint repairs,
but this can be restricted by using a narrower fan
pattern than that used for spraying a whole panel.
When this is done, a higher volume of paint will be
applied, increasing the risk of runs, and so the fluid
needle adjusting screw should be turned to the
right until a satisfactory volume of paint issues
from the fluidnozzle.
Occasionally the spray painter may be called
upon to repaint a motor car on which the paintwork
is in excellent condition, the customer simply
desiring a change of colour. In this case he may
only require to wet flat the surface with P600 grade
paper using a weak solution of water miscible
cleaning solution to remove any wax polish. Then
following thorough drying off and masking up,
three coats of half-hour enamel will produce quite
a good finish which can be further improved by
flatting, burnishing and polishing.
Acrylic lacquer
Though cellulose synthetic finishes are best left to
dry in their own time to obtain best results, acrylic
lacquer can be force dried without damage to the
paint film, with a consequent speeding up of the
process. This can be done with infrared lamps or in a
heated booth (though not in excess of 50 °C). The
filler materials possess better build and flowout than
the cellulose-based materials, thus providing better
surfaces for the finishing coats. The spraying viscosity
of the finishing enamel is more critical than the
half-hour enamel, and only the thinner recommended
by the paint manufacturer must be used. Solvent
evaporation from the wet paint film is governed to
some extent by the workroom temperature, and
should this be below 15 °C a special quick repair
thinner should be used. This thinner evaporates very
quickly but tends to produce a low gloss which will
require burnishing and polishing. However, should
the ambient temperature be 15 °C or above, a good
hard glossy finish can be obtained straight from the
gun which does not require polishing. As acrylic
primer fillers have good adhesion properties the use
of etch primer, though recommended, is not essential
provided that the metal substrate has been properly
prepared and treated with a phosphating solution.
17.14.6 Acrylic lacquer: complete respray