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3Finishes
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THE CHOICE of any commercial vehicle body's finish depends upon:—
a) The material used in construction, lb) The life expectancy.
{c) The conditions encountered in service. Bodywork, finish is applied to preserve the public image required by the operator or his customer.
Probably the most important single factor affecting the selection of a finish is the repair potential. Selection of colour, for example, for fleets based in different parts of the country must not only be sound in colour fastness but must be capable of being reproduced by body refinishers in any part of the country.
Where a paint manufacturer's standard colours are selected, then subsequent colour matching is greatly helped when a small area of original colour is preserved. One way to do this is to cover a selected area with _a piece of plastics material sheet attached with tape. Possible suitable spots include the front skirt of a van behind the cab, down a door pillar face—especially in the case of insulated bodywork—or on the inside of a door behind the lining (or trim panel in the case of the cab).
Such a master paint area can be indicated by a small instruction label fixed to the vehicle cab and easily seen by a body refinisher.
The choice of paint finishing systems is extensive.
The fast-drying and tough properties of cellulose, when combined with the elastic properties of modern synthetic resins, produce a high-gloss straight from the spray-gun. And the gloss has good retention properties. This type of finish is equally suitable for metal or timber surfaces, and can be used for repair work over synthetic finishes.
When a livery incorporates a standard insignia or advertising symbol or name, this can be introduced at the same time as the finish is applied by the use of premasked transfers in which the self-adhesive transfers are applied while the body is being painted. When the final finish is completed, the masking is carefully peeled away to leave the symbol or writing in position and surrounded by a clean, sharp paint edge.
Commercial vehicles which have been purchased in full colour will usually be found to have been finished in a modern acrylic finish, which first came to prominence in 1963. Repairs to this class of finish have become progressively easier. Repair can be effected either with identical material, or with suitable refinishing systems, with or without suitable sealers, depending upon thp particular materials originally used. The best results are achieved where force drying or radiant-heat lamps can be applied to the repaired area. Polish and spray-dust removal is carried out with either the use of a rubbing compound followed by polishing or with a polishing cornpound.
With the steady development of glass-fibre reinforced plastics materials for vehicle bodywork panels, finish has become an important aspect, especially since this material can be coloured off the job.
However, when colour pigments are introduced as a final finish for mouldings, problems of matching arise when damage has to be repaired. The reinforcement fibres are roughed up to form a key for the repair work, and these show through the newly applied
colour resin mix, causing a light ring around the periphery of the repair. These laminates can, however, be finished with conventional paints provided that pre-treatment is carried out according to the paint manufacturer's instructions, arid the colour in the moulding will act as a very satisfactory undercoat colour.
The latest material for body construction is the colour-coated aluminium sheeting in which vinyl and acrylic paints are applied to the aluminium sheets by the manufacturer using a reverse roller-coating process. This hard baked-on finish is fully malleable and will accept cropping, drilling and punching operations without any flaking or breaking of the colour film. Sheets having a stain-resistant finish are used in bodywork exclusively for such loads as acid-type fruits and vegetables and meat carcases, since the finish is resistant to bloodand acid-staining.
For vehicle bodywork which has to withstand the effect of cargo spillage, care has to be taken when choosing the correct material for the finish, since finishes based on an oil-modified alkyd resin will usually be stripped away from the bodywork in a very short service life. Paints based on acrylic resins have a low degree of dirt retention, which means that they can be easily cleaned, and so the vehicle is maintained in sound condition outside, Chlorinated rubber-based finishes are characterized by outstanding resistance to acids and alkaline% thus providing sound protection. but a high degree of finish cannot be obtained. The emphasis here is on protection, rather than appearance. Two-pack polyurethane finishes give a high-gloss,. abrasiveresistant film, of high chemical and solvent resistant properties, but these materials are the most difficult of all finishing systems to apply.
Light alloy panelled bodywork can be left unpainted, and where this type of finish is considered, then embossed sheet now available in several patterns is used for side panels and rear doors. This finish breaks up the normally bright, metallic appearance of the cold-rolled sheet and reduces glare from reflected light. The advantages of such finish is that slight impact scratches and dents are less evident than is the case with damaged paint-work.
Orbital shading of light alloy panels produces a finish having the same advantages as embossed sheet. While this type of unpainted finish requires that the surface be washed down regularly with a soapy solution and well rinsed with cold water in order to maintain a good appearance free from black marks, clear lacquer can be sprayed on to preserve the original surface finish. This lacquer is based on acrylic resin and has excellent adhesive properties on well cleaned and prepared alu minium surfaces. The lacquer is tough and reasonably flexible so that peeling is not a problem. Normally this lacquer must be applied by spraying, and the system requires a minimum of three coats applied wet-on-wet.
4 Body
maintenance
LOOKING over numbers of vehicles which have recently been brought into bodyshops for repair, it is evident that many features require regular attention if they are not to be the cause of failure at the testing station of the future. First there is the all too prevalent fault of jagged edges of wings, and damaged lower body sides having jags which could injure passers-by with resulting third-party damages claims. A cab damaged in such a manner as to impair driver control of the vehicle should be immediately repaired.
Taking a consensus of body damage due to poor maintenance, the first essential is the adequacy of underframe /chassis mountings. A diameter U bolt cannot be expected to carry the maximum load induced by the combined forces of maximum-braking effort and load weight;add to this the vibrations set up throughout the vehicle and these are the factors which are instrumental in creating U bolt fatigue. To these strains and fatigues must be added the heavy impacts and stresses caused by pounding over rough surfaces.
Except for light commercials, therefore, it is of practical advantage to replace all diameter U bolts with lin. diameter bolts or with hexagon headed bolts and flat plates.
The cumulative effect of a number of defects creates a situation where the vehicle might be considered as dangerous when loaded. For example, in a Luton or pantechnicon body, broken slats would not be considered as a defect, but if adjacent pillars or stringers were loose or suspect, then the body could be considered as unsafe with a load.
Body hardware such as drop-side and tailboard retainers and hinges need periodic inspection and replacement. When these are broken or distorted, the side or tailboard could become detached under the pressure of a load. Around the wheel arches, broken framing can be detected by bouncing the rear end of the body. This is done by using a long length of timber to prise the rear corner. If the inner panel of a wheel arch appears loose on the surrounding framing then damage to the supports at the cross-bearers is likely. In the case of light alloy planking which is fitted with "make-up" sections of timber along the edges of the floor, these can become detached from either the body side or drop if the underframing is damaged.
Similarly, if a number of rivets securing the light alloy panels to the framing are missing, especially towards the top of the body, this indicates a general loosening of the frame joints.
Examination of the joints of headboard and tailboard top framing-members will show up in transverse movement of the body, this creates badly fitting doors or roller shutters at the rear of the body.
Drivers' cabs are always suspect, and doors which will not close, sliding or drop windows which do not function, can be considered as elements likely to obstruct driver-control. Cab mountings which have rotted away or loosened need attention and in the earlier designs of tilt cab particular attention needs to be paid at regular intervals to the operation of the mechanism. There is a tendency for this to rust and obstruct the automatic return of the securing clip or hook.
Mountings for the seats in the cab need periodic checking and tightening. So do the bolts or studs on the underside of the cab floor. Rust takes a hold on the area adjacent to the bolt holes, and the seat can give way under the heavy pressure of emergency braking, this occurs at the bolts or studs which are torn from the floor.
Where such a condition prevails, the floor/ cab hull joints should be checked, because the area round the base of the cab may also have rusted away. This is especially so around the door sill stiffener sections or around the wheel arch of a forward control cab. Where sections of a cab have parted company, temporary repairs are often effected with strips or short planks of wood bolted through the metal to form a joint.
Such a practice would be suspect on inspection—but the final result would depend upon the actual location of the repair, and whether or not the joint had been made in a manner similar to that used in the construction of a composite cab construction. For example, if a cab front panel has been damaged, and the original panel cut away, a timber framing made up and the new panel secured to this framing, provided this in no way affected the security of the cab to the chassis such a repair would not be considered as a defect. But if a piece of board has been bolted over a hole in the floor as a temporary repair it is likely to be frowned on if only on the basis that it might prove an obstacle over which the driver could trip on entry or exit.
The under-door sills of steel cabs are prone to rotting away and this might be considered as a defect if the corrosion is sufficient to permit any movement at all of the front-to-rear cab structure, normally tied together by these sills. Similarly, where door steps are torn away this would be considered as a serious fault.
If sheet steel repair on unstressed parts is carried out with one of the proprietary resin materials, this would normally be considered as satisfactory especially where the use of this material is extended to give protection to adjacent areas of steel.
Platform and dropside bodywork suffers from broken timber cross-bearers, or in the case of light alloy, from outrigger straining, making these parts necessary for close inspection before submission. Similarly, head board fixings will need attention especially if fretting is evident around the fixings, together with half-round or capping metal strips, and tarpaulin rope securing hooks. Broken or missing hooks will be a fault because when a tarpaulin is being used it would be free to blow about in the unsecured area. Where spare wheel carriers are fitted the condition of the carrier and its mountings together with the method employed to secure the heavy spare wheel must all be in good order . . . whether or not a spare wheel is actually in position.
Although the lamps might normally be considered as a part of chassis maintenance the statutory lamps are fitted by the bodybuilder in the case of specialist cabs and bodywork, and so attention must be drawn to the mount ing of all lamps. Together with electrical con nections, particularly where the wires pass through holes in metal bodywork, these form part of the body. See that rubber grommets are in position, since bad connections or frayed wires, leading to unlit or flickering lamps, could be cause for rejection.
Finally the mountings of registration plates —both front and rear—need to be checked for security. Where these plates are bolted directly to steel bodywork this provides a water trap and subsequent corrosion, which in turn will allow the fastenings of the plate to rust away so that the plate becomes dislodged or falls off.