TIME TC PAINT?
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Modern Painting Equipment and the Correct Finishing Procedure Can Save an Operator a Great Deal of Expense and Reduce Time off the Road Substantially
by Richard Dangerfield
IN pursuit of catching the customer's eye, whenever and wherever possible, the commercial vehicle operator should not forget that the vehicles he runs are the ambassadors of the company and in them there is a first-class advertising and public relations medium. Not only should his vans and lorries carry goods, they can also carry a message, in paint, to sell the goods.
Many operators now paint their own vehicles when new, or carry out repaint schedules every 18 months to two years, or even do both. In the past few years this has been made easier, as modern finishes have been formulated so that they are relatively easy and quick to apply, fast-drying, require a minimum of skilled labour and produce coatings of a high standard of appearance and durability.
Recently, there have been many developments in surface coatings, but commercial vehicles are still generally finished in one of three types of paint—cellulose, air-drying synthetic enamels, or half-hour synthetic enamels.
Cellulose enamels, because of their greater cost, critical application conditions and the higher labour costs involved in final polishing, are usually impracticable for operators whose main interest is to get the vehicle back on the road as quickly as possible. The main properties of this type of finish, which is by spray gun application only, are hardness, durability, high gloss and long life.
However, as cellulose dries quickly, it is inadvisable to use it on large vehicles. Neither should cellulose be used where a body is fabricated from panels, as the paint tends to crack where flexing occurs at the edges of the panels. Where regular maintenance repainting is carried out and the speed of the process is an important consideration, a synthetic finish, which will look well for about two years, is often acceptable. Systems comprising only a primer, or in some cases a glossy primer-surfacer are, therefore, suitable. The primer-surfacer needs only a light rubbing down before applying the high-gloss finishing coat, either by brush or spray, which is dust-dry in two to four hours, according to the make of paint used.
With this type of finish, which is based on air-drying or oxidizing alkyd resins, clean working conditions are essential because of the time taken to reach a dust-dry condition. A considerable amount of dirt and dust could be picked up under adverse conditions. Paint shops must, of course, be as clean as possible whatever material is being used, but where it is not possible to produce absolutely dust-free conditions required with an air-drying synthetic, the halfhour synthetic should be used. This is a combination of cellulose and synthetic enamels applied by spray only; and provides a high gloss without polishing and is sufficiently quick-drying so as to avoid dust collection.
Where a finish is required that is resistant to chemical reaction, such as in tankers, then a paint based or chlorinated rubber or epoxy resins should be used.
A close examination should always be made of all work for refinishing and repair, and a proper assessment made of the job involved, the preparation of the surface, and the types and qualities of materials necessary. The amount of care and attention taken in the initial stages will add to the life and appearance of any paint film.
Some painters are in the habit of running their hands over the clean metal surface to make sure there is no rust or moisture present. This is a bad habit as it tends to transfer oil traces from the skin to the metal. A wise precaution is the use of a wax and grease remover, which will ensure the proper adhesion of the priming material.
If there is any rust on the metal, or even a suspicion of rust, no priming should be done until the metal has been treated with an acid that will neutralize the rusting action. Several solutions are available, which can be applied at ordinary shop temperatures and can be washed off with water after 20-30 minutes. They also have a useful etching action which provides for maximum adhesion to the metal.
If any soldering has been done, the soldering acids must be neutralized with a mixture of equal parts of ammonia, alcohol and water. With a cellulose finishing system it is essential to have an absolutely rustand wax-free surface before proceeding with the painting. An initial primer-surfacer, which is nitrocellulose resistant, should be applied by brush or spray and left to dry overnight. This should be followed by a coat of cellulose primer-surfacer thinned 50 per cent. and after 20 minutes indentations in the surface can be filled up with stopper, which should be given 30 minutes to dry.
Two further coats of primer-surfacer should be applied and left to dry for at least four hours after the last coat. The surface should then be rubbed down with abrasive paper grade 360 and thoroughly cleaned and dried.
Spray three or four coats of 50 per cent. thinned cellulose finish allowing 20 minutes between coats. This should give a first-class commercial finish, but if slight sinkage is noticeable, or an extra high quality is required, allow the surface to dry overnight, wet-flat, apply a further double pass coat of colour and polish.
The long-term durability and considerable off-the-road time involved wish cellulose finishing may not be practic
able to some operators, and a system involving only a primer is more acceptable. The high-opacity air-drying synthetic finishing coat requires no laborious flatting down and a large vehicle can be. finished in two to three days.
The surface to be painted should be cleaned with oil and grease remover and treated with derusting solution, washed down and dried off. One coat of coach finishing primer should then be brushed or sprayed on and left to dry for a minimum of 16 hours. An optional light dry de-nib with grade 220 or 240 abrasive paper may then be used, followed by a making good of any damaged parts with knifing filler or stopper, allowing overnight drying after the final application.
Wet rub with 220 or 240 abrasive paper and, when dry. apply one coat of synthetic groundcoat. This should be dry in four to six hours if conditions are good. The first coat of coach finish can then be brushed or sprayed on and this may be wet-flatted after a minimum drying time of 16 hours. The final coat should then be applied and left to dry foiat least 24 hours before putting the vehicle into service.
• The half-hour synthetic finish should be used where quick-drying is required. This is a compromise between the straight cellulose and the pure alkyd finish, providing a good gloss and normally drying in half an hour. Using this syste.m, a good finish will be obtained on new work by cleaning down the metal, chemically or manually etching the surface, spraying two coats of cellulose primersurfacer and, finally, two coats of thinned finish sprayed at half-hour intervals.
• A top-quality coach finish can be achieved by the addition of a zinc chromate primer, before the primer-surfacer. and three tcip coats instead of two, flatting with abrasive paper before applying the final coat.
On glass-fibre bodies it is essential that all traces of the mould release agent should be removed, otherwise paint adhesion will be affected. The agent may be either a watersoluble type or a wax, and can be effectively removed by a thorough cleaning with water or a hydrocarbon solvent such as white spirit. This should be followed by a flatting down to ensure adhesion of the primer-surfacer coat.
With aluminium bodies it is recommended that they be given an application of a chemical cleaner, followed by etching primer, or a thorough clean and scuffing with abrasive paper.
Complete varnishing is carried out over air-drying coach c I I finishes. On cellulose finishes, only pendi varnishing of lettering is normally undertaken. Whether the painting process is completed with a coat of varnish depends on the quality of finish required. Varnish does give added protection and, in some cases, improved appearance. It should, however, always be applied over lettering and special designs. Varnish is not normally necessary except with rich blues and maroons, when a coating will preserve their high standard of gloss. It is recommended that two coats be applied, the first being well wet-flatted with 320 abrasive paper. Where a vehicle has to operate in tropical or semi-tropical climates a glazecoat, consisting of a mixture of three parts of coach finish with one part of varnish should replace normal varnish coats. Line writing and transfers should be protected by pencil varnishing.
On repaint jobs due regard to sources of trouble must be given attention. The first thing to determine is whether or not the old paint has good adhesion. If the film shows signs of blistering, cracking, or crazing, and so on, the best thing is to strip off the old film with paint remover and treat the bare metal with anti-rust solution. To check the adhesion properties, sand through the finish in several places and feather-edge a small spot. If the thin or feathered edge does not break or crumble, it is reasonably safe to assume that the old finish will "stick to the job."
Another precaution should be to make sure there is no more old wax or silicone polish left embedded in the finish. Unless all traces of these polishes are removed, the drying of synthetic enamels may be considerably slowed down, destroying one of their principal advantages. A wax and grease remover or water-miscible cleaning solution should be applied to the surface and wet-flatted with an abrasive paper, using the cleaning solution as a lubricant.
As cellulose paints contain strong solvents, they can act as very efficient paint removers when applied to old finishes, and the third step is to determine whether the finish is cellulose or synthetic enamel. Old cellulose films can be dissolved by spraying a test area with cellulose thinners and rubbing it with the hand. If the finish is left impervious and does not come off on the hand, it is likely to be synthetic enamel.
If, however, the finish proves to be cellulose, or any doubt arises, precautions must be taken to prevent swelling c12 of the old coat when new cellulose material is applied. Sealer or barrier coats based on air synthetic resins are especially designed for this purpose and they have good adhesion to old surfaces and prevent the penetration of solvents contained in subsequent coats.
Synthetic solvents are relatively mild and all that is needed is a light rubbing down with a fine grade of abrasive paper to make sure that the whole of the surface is thoroughly and evenly sanded to " key " the new finish well onto the old surface.
Modern methods of application can reduce maintenance costs and off-the-road time substantially. Hot spray, for example, although requiring a greater initial capital outlay (between £100 and £150) is an excellent proposition for medium and large commercial fleets. The principle is that the application of heat reduces the viscosity of paint to spray consistency and eliminates the need for large quan tities of thinners. The advantage is that the paint is sprayed at a higher solids content, which permits a heavier film applied in a single application.
Less expensive equipment is also available for heating paint. The electrically heated cup method, for example, may not maintain the paint at a Meady temperature, because of normal heat losses, and overloading of the paint may result; consequently shrivelling will occur.
Three Ways of Feeding
There are three main ways of feeding the paint to the spray gun—under pressure, by syphon or by gravity. The usual method of applying paint is by high-pressure gun, when the paint is atomized with a blast of air at a rate of approximately 60 p.s.i. The air breaks up the paint and blows it out from the gun nozzle. By this method the paint flow can be easily and quickly adjusted, but a disadvantage is that there is always a great deal of wastage with an accompanying fog of paint in the spray shop.
Where working conditions are cramped and extractor equipment is neither practical nor efficient, the airless spraying process is a useful method to be investigated. The principle is that the paint is pushed out under pressure through an orifice, which causes it to atomize. Because of the lack of high atomizing air, the paint is not able to swirl about and consequently there is much less wastage and comparatively little paint fog. A disadvantage with this method, compared with high pressure air spraying, is that there is no means of controlling the delivery rate whilst spraying. Variations can only be achieved by stopping to change the nozzle.
A more expensive means of finishing on a large scale is by an electrostatic hand gun. A large voltage is applied to the paint, which is pushed out on to a spinning head and is thrown to the edge by centrifugal force. It is then atomized from the head and, through the action of the electrostatic field, the particles are drawn irresistibly towards the object to be coated, which is earthed.
This is a relatively slower method of application, and, of course, with no wastage of materials. It produces a smooth, constant coating with only a few awkward places such as deep interior angles which are incompletely painted and need to be touched up with a spray gun.
Small parts such as seat frames are more economically painted if they are dipped in the paint and articles such as nuts and bolts can be tumbled in a wire-mesh barrel immersed in the required finish.
Painting from new or refinishing is not in any way an easy matter. It calls for a great deal of care and skill at all stages, but of prime importance is the amount of attention given to the early stages of finishing. Herein rests the high standard of appearance and a long-lasting paint film. Many jobs for respraying have old and weather-beaten finishes. Fine cracking or other failures are often undetected or lost by the preliminary flatting or cleaning operation, and are apparent only after the active solvents of the refinishing coat are applied, causing the old finish to lift up or wrinkle.
The adhesion of paint to a surface is the most important single factor determining the life of a paint film. Poor adhesion will result in a rapid breakdown by lifting, blistering, pinholing, running, or many other film deteriorations. Detailed here are some major defects with their causes and remedies.
Lifting is a puckered or wrinkled effect usually resulting from -applying material containing strong solvents over a partially dry cellulose coat. The strong solvents in the new finish dissolve the old finish in places and open up cracks and defects.
Susceptible to Solvents
Oil-type enamels used on some commercial work are very susceptible to lifting by strong cellulose solvents. Swelling of sand scratches is also met occasionally, which is a very mild form of lifting. It can also take place in applying synthetic enamel over an inadequately cured synthetic film. Application and thinning have much to do with this.
Where lifting has occurred, it is usually necessary to strip down to bare metal and refinish completely, but if this type of defect is superficial it can sometimes be flatted out and repaired with a primer-surfacer. The finish should be well thinned and sprayed in mist coats, with ample drying time between coats. Heavy wet coats, which can soften the whole paint film should be avoided.
Blistering usually occurs when paint is applied over a surface that is covered with hard resins, oil, grease, wax or silicone. A running effect of the new film will be observed. Poor flow-out, lack of adhesion, inadequate wetting and small craters are accompanying symptoms.
The remedy is to strip to bare metal by sanding and by giving a thorough clean with wax and grease remover. The surface should be thoroughly water washed and dried off with clean rags and air to blow out crevices and mouldings. The process should be repeated if necessary.
Orange peel is a pebbled or dented superficial effect of
the finish which, as the name suggests, resembles the skin of an orange. It is a frequent complaint resulting from a number of causes, the main ones of which are unsuitable or insufficient solvents, incorrect spraying pressure, adverse application conditions or insufficient weight of coat.
A thinner which dies too quickly or has inadequate solvent strength will produce orange peel. Cellulose material sprayed on a hot surface will show orange peel due to over-quick drying.
Time should always be allowed for proper flow-out, and a good thinner, as recommended by the manufacturer, should be used. Where orange peel has occurred, this must be wet-flatted with 320 grade abrasive paper, when a properly thinned atomized material may be sprayed on, paying particular care to the handling of the gun. .
The defect knownas pinholing is self-explanatory and is usually caused by spraying the paint too dry, especially with undercoats. It can also be due to too heavy an application of the finishing coat, particularly under warm conditions. This may be due to incorrect spraying or to the material being at too high a viscosity.
• Where the paint has been sprayed too dry, the top surface must be removed by wet-flatting and a coat of primersurfacer applied. Heavy colour coats should also be wet flatted. Under persistently warm conditions the addition of a retarder to the thinner used in the spraying mixture will increase flow and may eliminate pinholing. Additions usually found effective are 5-15 per cent. of the thinners.
Paint normally runs or sags because of an application of too heavy a coat of material. This usually results from incorrect thinning or holding the gun too near the surface when spraying. The defective area should be removed by flatting with wet 320 grade abrasive paper, washed and dried. Respray with one or two coats of the appropriate material and continue with the process in use.
When the humidity is high in the -paint shop, the colour coat will sometimes turn misty. This is usually termed blushing. If this occurs on the first colour coat, further coats may be improved by the use of a retarder to replace from 5-15 per cent. thinner.
The type of finish and the method of application are not matters to be decided without expert advice in the light of each individual circumstance. If there is any doubt or problem concerned with finishing, many paint manufacturers themselves give an excellent service in this respect, and maintain technical advisory departments of which fell use should be made.
Some of the information for this special article was obtained with the kind co-operation of Imperial Chemical Industries, Ltd., Points Division, and General and Industrial Paints, Ltd,