Cellulose synthetic (half-hour
enamel): complete respray from
Bare metal
The process summary is as follows:
1 Spray a thin coat of etching primer, mixed 1:1
with activator, thinned to 20–25 seconds
viscosity.
2 Allow to dry for 15 minutes.
3 Spray cellulose primer surfacer, thinned 1:1 to
19–22 seconds viscosity. Apply three full
coats, allowing 10–15 minutes between coats.
4 Allow to dry for 1 hour.
5 Wet flat with P400 or P600 wet-or-dry paper.
6 Apply cellulose stopper where necessary,
allowing 15–20 minutes between layers.
7 Allow to dry for 1–1 hours.
8 Wet flat stopper with 320 wet-or-dry paper.
9 Spray cellulose primer surfacer to stopped up
areas, and flat with P600 grade paper.
10 Blow off vehicle with air gun and tack off.
11 Spray finishing material thinned 1:1 to a viscosity
of 21–23 seconds. Apply one coat and
allow to dry for 15–30 minutes. Apply second
coat.
12 Allow overnight drying.
13 Wet flat with P800 grade paper, dry with air
gun, tack off.
14 Spray double-header coat, thinned as before.
Alterations to this standard method can be made
where necessary. For example, an alternative to
using cellulose primer surfacer at stage 3 would be
to use a synthetic resin primer surfacer which has
better build qualities and may be preferable on
rough surfaces. It is thinned 4:1 giving a viscosity
of 26–30 seconds. Spray two or three coats, allowing
a flash-off period of 30 minutes between
coats, and the surface will appear completely matt.
Allow the final coat to dry for 4 to 6 hours before
dry scuffing, or to dry overnight before wet flatting
with P600 or P400 grade paper. Cellulose
stopper cannot be applied between coats of
this material, and when used after the third coat
must be oversprayed with cellulose primer filler.
Cellulose finishing paints can be applied directly
over synthetic resin primer surfacers, but if drying
conditions are less than ideal a certain amount of
lifting of the surfacer could be experienced. This
is caused by the strong solvent used in cellulose
finishes penetrating the still soft synthetic primer
surfacer and acting as a paint remover. Therefore
when adverse drying conditions prevail, such as
low temperature, dampness or high humidity, it is
advisable to spray a coat of sealer or cellulose surfacer
over the job before applying the cellulosetype
finish.
At stage 11 in the process, and alternative
method is to apply one coat of the finishing paint,
allow to dry for 15 to 30 minutes, then spray a
double-header coating. This is a quicker method
than that mentioned in the process summary, but
does not produce the same standard of finish.
It should be noted that with the exception of
tacking off and the actual application of the paint,
all the process stages should be done outside the
spray booth. Cleanliness within the booth or spray
room is essential if high-class finishes are to be
obtained. Rubbing down, even though done with
water, will leave a scum in the workshop floor,
which when dried out will leave behind a powdery
604Repair of Vehicle Bodies
residue. When disturbed this will cause air contamination.
Burnishing and polishing cloths also cause
contamination of the atmosphere, and these
processes should be carried out in a part of the
workshop away from the spray booth. These polishing
cloths must not be put down on the floor or
dirty work benches, for if grit should be picked up
on them, damage to the new paint finish will occur.
As to the correct air pressure to use when spraying,
this will vary according to the type of spray
gun being used, the paint viscosity, and the type of
paint. For cellulose synthetic paints and air pressure
of about 4 bars (60 psi) is normal, though this
can be adjusted by the spray painter to suit his own
requirements. The golden rule is to use the lowest
air pressure that will give satisfactory atomization.
Stage 6 in the process summary refers to the
application of cellulose stopper. It should be borne
in mind that this material dries by the evaporation
of the solvent content, and if it is applied in heavy
layers the inside of the coating will remain wet for
a considerable period. In addition to this, when
drying does eventually take place the stopper will
contract and sink. A much more satisfactory repair
will be obtained if the material is applied in thin
layers with a suitable drying period between applications.
Plastic spreaders are to be preferred to
metal knives when laying on the stopper, as these
cause less damage to surrounding areas of paintwork.
The lid must always be firmly replaced on
the tin or tube of the stopper when not in use, otherwise
the stopper will harden in the container and
cannot be satisfactorily resoftened.
A coat of sealer could be added to the process,
where costs permit, between stages 10 and 11. This
would reduce absorption of the finishing coats by
the surfacer and promote a high gloss.
Following the finishing coats, the vehicle may, if
required, be burnished and polished; the method
for this is described in Section 17.15.
Finally, it should be borne in mind that the use
of a guide coat at each of the stages of sanding
down is invaluable for locating surface defects.