In the repair paint shop there is a range of painting tools which make the work of the painter easier and improve the quality of the repair paintwork.
The spray gun is the most important implement in the paint shop. Application of paint using the spray gun can produce a layer with absolutely constant thickness and a smooth paint surface.
Because of the construction design and with the aid of compressed air, a spray-ready paint mixture is dragged out of the container to the nozzle by the venturi effect, and is applied to the surface being worked.
When the trigger of the spray gun is pressed to the first pressure point, only the compressed air passage opens. If the trigger is pressed further, the nozzle needle displaces and the air stream drags paint with it at high speed. This produces a spray mist consisting of micro-droplets of paint.
Types of spray gun
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Suction-beaker spray gun
In the flow-beaker spray gun, the paint container is mounted above the spray gun. On the suction-beaker spray gun, it is below.
Furthermore, spray guns are categorized by their air pressure requirement into high and low pressure guns.
High pressure guns have the disadvantage that they exhibit high consumption of energy and materials. The spray pressure they require is between 1 - 6 bar.
Because of the high air pressure and the large amount of air needed, the result is a powerful paint mist formation (paint transfer rate approx. 35%).
Current practice is mainly to work with reduced mist spray systems (RP and HVLP systems).
Reduced pressure (RP) guns are optimized high pressure guns which have an input pressure at the gun of approx. 2.5 bar and an atomization pressure at the air cap of 1 - 2 bar. In practice this spray technology is preferred for spraying clear lacquer because of the finer atomization.
Low pressure guns have the advantage that they exhibit minimal paint mist formation and because of this the paint transfer rate rises to approx. 65%. The spray pressure required in this case is between 1 - 5 bar. Nozzle sizes from 1 - 2.2 mm can be used.
HVLP spray guns
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Working pressure control
The high volume low pressure (HVLP) spray gun is a high performance spray gun which forms a soft, fine and homogenous spray pattern. The atomization pressure at the air cap is 0.7 bar when the input pressure at the gun is 2.0 bar.
The low atomization pressure of 0.7 bar together with greatly reduced spray mist provide high material ejection. The low nozzle internal pressure minimizes rebound of the paint droplets from the object and thus the proportion of overspray.
This spray technology has a very high application efficiency. By matching the size of the nozzle, the HVLP spray gun can be used for all repair painting materials.
HVLP spray guns are often used in practice for the application of water based paints.
Mini spray guns are often used for small, localized touching-up work. Use of HVLP spray technology and nozzle sizes of 0.3 - 1.2 mm permits very fine work, so that the area of the repair can be kept as small as possible.
In order to ensure that a spray gun operates efficiently for a long time, careful cleaning is absolutely vital after use.
NOTE:During cleaning you must distinguish between water based and solvent based materials.
Cleaning by hand:
- Empty the paint beaker immediately after use.
- Flush the gun with cleaner.
- Clean it inside and outside with a brush.
- Dismantle the gun to clean it thoroughly.
- Clean the air cap using a suitable brush.
- Use nozzle cleaning needles to clean bores and nozzles.
A spray gun washing machine is recommended if the painting work is highly intensive.
New types of paint processing systems are replacing the conventional mixing beaker, filter and spray gun flow beaker. This reduces the amount of solvent required for cleaning and the amount of routine waste which remains.
Paint preparation system (PPS)
With this system, which is suitable for both suction and flow beaker spray guns, only one beaker is required for mixing and painting.
A bag is inserted in the beaker, in which paint can be mixed, processed and stored after use or completely disposed of.
The small quantity of paint remaining in the gun is removed using a minimum quantity of solvent from the pipette bottle.
The amount of cleaner used is reduced because only the spray gun needs to be cleaned.
Hand and machine sanding tools
Sanding is used to prepare a surface for application of a paint layer, enabling it to adhere well. Sanding materials have a great influence on the quality of a repair paint finish. The correct sanding medium must therefore be chosen for every material.
During sanding, material is mechanically removed from a surface.
In the paint shop, carborundum or silicon carbide abrasive on a substrate of paper or cloth are the most common sanding materials used.
Carborundum is a very hard mineral consisting mostly of aluminum oxide. During use carborundum becomes blunt and wears away.
Silicon carbide has a very high degree of hardness, but is more brittle than carborundum. When silicon carbide is used, the mineral grains break. New long and pointed profiles are formed.
Use of the correct sanding paper depends on the application, the substrates and the tools used. The following table can be used as a guideline, but the recommendations of the supplier of the auxiliary materials and additive materials must be followed.
Soft Pads are recommended for manual refinishing of contours, curves and difficult to reach areas. On a Soft Pad the abrasive is found on a coarse structured fleece. Because of this, it is very flexible, does not kink and does not slip in the hand. This enables a fine and even finish to be achieved.
The disadvantage of electrically driven machines is that their own weight is high compared with pneumatic systems. They also become warm during work. They do not however need any special operating equipment for their energy supply.
Sanding machines are categorized by their type of sanding movement.
Rotational sanders
On these machines the sanding paper turns.
- Advantage:
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Ideal for heavy sanding work.
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Fast and aggressive sanding possible.
- Disadvantage:
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Large amount of heat developed.
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Difficulty sanding flat surfaces.
- Application:
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Removal of old paint layers.
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Preparation of panel for stopper.
Oscillating sander
On these machines the sanding paper oscillates. The backing pad is rectangular.
- Advantage:
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Ideal for large and flat surfaces.
- Disadvantage:
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Hardly useable on rounded surfaces.
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Flexible backing pad not possible.
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Vibrations because of the poor support of the backing pad.
- Application:
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Sanding of polyester stopper.
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Sanding processes on flat surfaces.
Orbital sander
On these machines the sanding paper turns and oscillates.
- Advantage:
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Easy to handle and good sanding power.
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Minimal heat development.
- Disadvantage:
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Not suitable for sanding stopper on flat surfaces.
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Smooth guidance important, otherwise sanding marks will occur.
- Application:
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Sanding of paint layers.
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Well suited for final preparation of a primer.
NOTE:Comply with the manufacturer's recommendations when setting the orbital sander.
On the orbital sander, stroke settings of approx. 3 mm for fine sanding work and approx. 5 - 7 mm for coarse sanding work have been established.
Polishing and finishing tools
The term polishing in the context of paint repairs means the elimination of paint flaws and high shine polishing of neighboring parts.
During polishing the fine sanded surface is returned to a high shine using a special abrasive polish.
Before the actual polishing, all flaws in the paint surface must be removed and the following working procedures must be adhered to:
- Thoroughly clean the vehicle.
- Remove spray mist from all surfaces.
- Sand out and polish particle inclusions.
- Sand down paint runs and polish them out.
- Examine the exactness of the color match in daylight.
- Remove masking edges.
- Remove sanding water, sanding dust and polish residues.
After the polishing process the results must be tested using a special test spray.
Infrared drying technology
The drying process in a painting/drying cabin occurs through heat conductance (convection). When an infrared dryer is used, the drying process is through heat radiation.
The infrared rays penetrate the air and the paint layer without warming them. Because the infrared rays are reflected from the steel panel, the paint coat is warmed from the inside outwards.
Advantages of infrared drying:
- The drying process occurs from the inside to the outside.
- The drying time is shorter than for warm air systems.
- Because the infrared dryer consists of several cassettes which can be switched on independently, the drying area can be optimally controlled.
Independent of the manufacturer's instructions, pay attention to the following:
- Flash-off time of the paint before switching on the infrared dryer.
- Distance between the infrared dryer and the surface.
- Duration of the irradiation.
The most common use of the infrared dryer is to dry stopper and primers. The wait time between the job steps is shortened without having to use the painting/drying cabin.
The painting/drying cabin can then be used exclusively for application and drying of topcoat.
There are two types of infrared dryer:
- Infrared dryer with short wavelength radiation.
- Infrared dryer with medium wavelength radiation.
As an indication, the following drying times are listed for some materials (at 80 cm distance):
NOTE:Observe the material manufacturer's and supplier's specifications.
- Polyester stopper 2 minutes.
- Spray stopper 2 to 7 minutes.
- Water based primer-filler 7 to 9 minutes.
- Primer 3 to 8 minutes.
- Top coat 7 to 10 minutes.
Air dryers
The air dryer is suitable in places where drying needs to be done, but without great outlay (painting/drying cabin or infrared dryer).
NOTE:Air from the compressor is often too cold for effective drying.
Air dryers use the venturi effect to blow the warm ambient air over the paint surface in a gentle air flow.
Paint mixing system
Because of the many different color variants, it is now seldom possible to store all color shades as ready-made mixtures.
For this reason, vehicle manufacturers make the mixture proportions of their paints available as color codes. The required color shade can be obtained from the paint mixing system using this color code.
All the color components are combined according to their proportions by weight using a precise computer scales to produce a finished color shade.
Painting cabin
The air requirement in a painting cabin is large. The outside air which is drawn in must be passed through filtering and warming equipment. This particularly applies during colder times of the year and especially for combined types of building where the painting cabin is also used as a drying cabin.
It is primarily used to keep the air free of dust. At the same time, explosive solvent-air mixture concentrations are prevented
NOTE:Vacuum will lead to contamination of the newly applied paint. The outside air flows through door gaps, wall joints and other openings and as it does so, brings dust deposits with it.
The air supply quantity depends on the size of the painting space and the quantity of extracted air. Enough air must be supplied to cause positive pressure in the painting space. An air extraction : air supply ratio of about 1 : 1.05 is sufficient.
The filters should have a dust-removal grade of not less than 99.8% and must always be kept clean.
It is especially important that the air supply does not cause strong air currents in the painting cabin. If not, the following problems could occur:
- Paint contamination cause by paint mist, which persists in air eddies and gradually falls on the fresh paintwork.
- Flow problems in the paint because of the high speed of the air, causing the paint to thicken very quickly on the surface.
- Loss of gloss and wrinkle formation because the surface dries too fast.
- Painter disturbance while working.
In modern paint cabins the air supply is provided from the complete surface of the ceiling. The air speed should be 0.3 m/sec (measured in the unrestricted cross-section of the spray cabin). At the same time, the air in the cabin should change about 350 times per hour.
Air extraction is best achieved through extraction channels in the floor of the painting cabin.
NOTE:Refer to the manufacturer's specifications for the operating instructions, safety instructions and notes on the maintenance of a paint cabin.
Smooth walls in the paint cabin should prevent dust deposits. Regular cleaning is necessary however.
Special easily washed adhesive-bonding paint can be applied to the walls to protect the cabin from paint mist.
