Quick air drying synthetics

This type of material offers special advantages for

low-cost resprays and rapid finishing of commercial

vehicles. The main advantage is that it is suitable for

application over all types of existing finishes, and is

suitable for hot or cold spray application.

The rapid respray system is as follows:

1 Clean off all traces of traffic dirt, grease, wax

and silicone polishes by using water miscible

cleaning solution.

2 Feather edge all damaged areas including parts

damaged by stone chips (generally on sills).

Treat bare metal with derusting liquid.

3 Spot prime any bare metal areas with cellulose-

based primer surfacer, and allow to dry

for half to one hour. Stopping, if necessary, is

best done before priming with two-pack polyester

stopper.

4 Wet flat the whole vehicle with P600 grade

paper, using liquid detergent or water miscible

solution as a lubricant and to remove wax.

Rinse, dry and tack off.

5 Thin the enamel to a viscosity of 23–26 seconds

(4:1). Spray one coat and allow to flash

off for 10 minutes, then apply a double-header

coating. The finish may also be applied hot, in

which case it requires no thinning. The paint is

heated to a temperature of 70–80 °C, and one

coat will be sufficient to give the required film

thickness. The flash-off times between coats

recommended by the paint manufacturer must

be strictly observed. If too great a time lapse is

allowed, then problems of lifting or wrinkling

may be encountered.

Low-bake enamels

These are modified high-bake enamels rather similar

in composition to those used by motor vehicle

manufacturers. Though developed primarily for the

refinishing trade, they are in fact used by vehicle

manufacturers in the rectification of damaged or

faulty finished vehicles. One of the problems

encountered by the refinisher has always been to

obtain a close match to the original finish. When

exposed to sunlight some colours have a tendency

to fade, whilst a certain amount of discoloration

takes place in others; some whites, for instance,

tend to yellow with time. In addition to this, some

pigments combine very well with a synthetic

medium but not so well with cellulose-based vehicles.

As the latter have always been the most widely

used materials in refinishing, colour matching thus

presented problems. These would have been less if

refinishing shops were equipped with equipment

similar to that of the vehicle makers, allowing them

to use the same finishing materials. This, of course,

would require a tremendous capital outlay which

would be well beyond the reach of the refinisher,

and would in all probability never be recovered.

In an effort to improve refinishing techniques, to

reduce drying times and to obtain faster production,

low-bake enamels (i.e. paints that cure at

lower temperatures than the original high-bake finish)

were developed. Being similar in structure to

the high-bake material, these paints offer a closer

match in colour and texture than do cellulose synthetic

materials. Burnishing and polishing is seldom

required as these paints provide a good gloss

from the gun.

Equipment

An obvious requirement for the drying of low-bake

enamels is a stoving room, and various types are

available. A stoving room can be acquired which can

be connected to an existing spray booth, or where

workshop space is limited a combined spray booth

and low-bake oven can be installed (Figure 17.52).

The obvious limitation of a combined spray booth

and low-bake oven is that no further paint spraying

can be done during the stoving schedule, in addition

to which valuable working time can be lost whilst

waiting for the booth to return to a comfortable

working temperature.

Where workshop space permits, a better proposition

would be to install a low-bake oven which can

be joined up to an existing spray booth with sliding

shutter doors to seal the two areas (Figure 17.53).

This can be further improved upon by having

entrance/exit doors at each end of the unit which

would provide a flow-line system of painting and

stoving (Figure 17.54), thus reducing time wastage

on vehicle movement. The type of installation must

obviously be governed by the size and shape of the

workshop (Figures 17.55–17.59). A further variation

Automotive finishing and refinishing 609

and have good adhesion properties when baked.

Following rubbing down and cleaning off, the surface

can be sprayed with synthetic resin primer

surfacer if it is of a rough nature. As previously

stated, these primers have exceptional filling properties,

but when used with low-bake finishes

the stoving schedule (1–1 hours at 82–93 °C) can

prove to be too long to be economical. Where the

surface is not too rough, a better method would be

to use cellulose primer surfacer thinned 1:1. Slight

imperfections should be levelled with cellulose

stopper, which must be over-coated with surfacer.

The surfacer can then be stoved for thirty minutes

at 70–80 °C. A third material finding favour is the

polyester primer filler described in Section 17.5.1,

which obviates the need for stopper on small

imperfections and provides a single coat build-up

to the finishing stage. As previously stated, several

coats can be applied wet on wet without flash-off

periods, and force dried for 15 minutes at about

75 °C. A further advantage is that polyester stopper

can be used, if necessary, between coats of polyester

primer filler. This material should be overcoated

with cellulose primer surfacer before

applying the finishing material. Whichever of the

above materials is chosen, the final coating must

be rubbed down wet, and following drying and

tacking off, the colour coats can then be applied.

Spraying

The finishing material is thinned 7:2 and sprayed

in one single coat followed by a double-header

coat, with a 15 minute flash-off between coats.

Another flash-off period of 15 minutes should be

allowed before stoving.

Preparation for stoving

It is current practice in the motor industry to repair

areas damaged during assembly with low-bake

enamels and then to stove the whole body fully

trimmed. The temperature inside the vehicle does

not rise sufficiently to damage the trim provided

that the doors and windows are properly closed.

Before stoving a vehicle refinished with low-bake

enamel, check that doors and windows are closed,

remove all exterior plastic fittings, increase tyre

pressure by 0.5 bar (5 psi), check that the petrol

tank is not too full and, if duotone work is being

done, remove any masking. It is not necessary to

remove the battery.

Figure 17.52Spray booth combined with low-bake

oven

is shown in Figure 17.60, in which the vehicle being

painted is moved sideways on rails and bogies.

Apart from the stoving room, the usual spray

painting equipment used in refinishing shops is

required.

Preparatory work

Deep indentations in the vehicle body can be filled

with polyester stoppers, which cure very quickly

610Repair of Vehicle Bodies

Stoving

The usual stoving schedule is 30–40 minutes at

80–100 °C, following which the body should be

allowed to cool for about 45 minutes (longer if

necessary) before further masking up for duotone

work is carried out. Certain parts of the car will be

sheltered from the heat, and consequently paint

applied to these parts will not be fully cured. A

converter liquid is available from most manufacturers

of low-bake paints which can be added to the

enamel for brushing in door edges and insides of

boot lids and bonnets, or these areas can be

Figure 17.53Low-bake oven joined to spray booth

Figure 17.54Oven joined to spray booth, with through doors for flow line

Figure 17.55Spraybake exterior units, height 3 m

(Spraybake Ltd )

Figure 17.56Multiple installation showing drive

through and paint mix room (integral) between units

(Spraybake Ltd )

Automotive finishing and refinishing 611

sprayed if the viscosity is reduced to 22 seconds

with the appropriate thinner. Any areas touched in

thus will dry quite hard at the normal paint shop

temperature.

Finishing

Polishing of low-bake enamels is not normally

necessary as a good gloss from the gun is obtainable,

but it should be borne in mind that these

paints remain more open, i.e. wetter, than more

coventional materials during the flash-off period

prior to stoving. For this reason, a clean workshop

is a necessity; otherwise dust may settle on the

Figure 17.57Multiple installation showing side

loading doors and track, and housing IRT arch

(Spraybake Ltd )

Figure 17.58Corner installation showing two

low-bake units, height 2.5 m (Spraybake Ltd )

Figure 17.59Interior shot of Green Booth designed

to meet EPA regulations (Spraybake Ltd )

Figure 17.60Joined oven and spray booth, with

sideways flow

612Repair of Vehicle Bodies

still wet film which will necessitate flatting and

burnishing to obtain a smooth finish. The nibs

should be removed with P800 grade wet or dry

paper and the flatted area brought up to a high

gloss with burnishing compound, followed with a

good-quality car polish. Low-bake finishing can

be carried out over both stoving and nitrocellulose

finishes providing they are in a sound

condition. Red or maroon cellulose finishes

which might bleed are best coated with a sealer

before commencing the spraying of the surfacer

coatings.

Developments

Apart from the cost of the stoving equipment, one of

the reasons why low-bake finishes were slow to find

popularity with refinishers was that it meant having

to increase the already extensive stock of paints normally

carried in refinishing shops. With this in

mind, paint manufacturers have developed a type of

thinner which, when added to their half-hour enamel

(nitrocellulose based), will convert it to a low-bake

cellulose enamel. Over 2000 car colours are available

in this material, and so colour matching does

not present too many problems. The cellulose stoving

enamel should be thinned with the converter

thinner to a viscosity of 23 to 26 seconds with a BS

B4 flow cup. Spray either three single coats or one

single and one double-header coat at 4–5 bars

(60–75 psi), allowing 15 minutes flash-off between

coats and another 15 minutes before stoving.

The stoving schedule recommended is 40 minutes

at 91–93 °C, during which the car body attains

a metal temperature of 80 °C for 30 minutes. This

temperature is not high enough to cause damage to

the car interior but is high enough to effect a complete

cure of the paint film.

Advantages

Low-bake refinishing, though costly to set up

initially, undoubtedly increases the potential productivity

of refinishing shops. Faster drying times

are obtained at almost every stage of the job, with

the result that vehicles are refinished much more

quickly than when using air-drying materials. This

leads to faster delivery of customers’ vehicles,

which is a very good selling point, and as the finished

article can be moved out of the working space

after cooling of the body, freer movement within

the workshop is obtained. Used properly, low-bake

finishes lead to a higher turnover of work, which in

turn increases the need for good organization

within the workshop (see Section 17.18).