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.