Paint spraying in vehicle body repair shops (bodyshops) is specialised because it usually calls for a mirror finish. To obtain the deep gloss required, the atomisation of the paint needs to be very fine and this is achieved with a compressed air spray gun. Unfortunately the air passing through the spray gun that atomises the paint, also blows it away from the surface being sprayed resulting in a large loss of paint as overspray.
For many years there was only one type of air spray gun for spraying vehicles, called a "conventional spray gun" that used compressed air at 40 - 60 psi pressure, which at the very best, could only achieve a transfer efficiency of up to 30% with at least 70% being lost. This is the % of paint reaching the surface being sprayed. Apart from the waste of expensive paint, the lost paint leads to premature clogging of the spraybooth filters that then require more frequent replacement (by an authorised contractor).
Over the last decade, a new type of air spray gun using a high volume of low pressure air has been introduced into bodyshops, called HVLP (High Volume Low Pressure). With the HVLP spray gun, the high volume of air helps to carry the paint particles to the surface being sprayed and a theoretical transfer efficiency of up to 65% can be achieved with it. In practice, a transfer efficiency of about 50% is obtained, depending on the work being sprayed. HVLP spray guns tend to require a larger volume of air than their equivalent conventional gun, although there is little difference in the most advanced designs.
The traditional paint supply for an airspray gun in the UK, comes from a paint cup of about 1 litre capacity hanging below the gun and in front of the trigger, often called a syphon cup, although there is no syphonic action, the paint being sucked up by the venturi suction generated at the point of atomisation of the paint: the paint nozzle or spray tip.
Truck spraying with 2 litre cup
An alternative paint feed that has gained popularity in recent years, is supply from a gravity cup above the spray gun. The choice between syphon/suction cup or a gravity cup is a question of preference by the sprayer. Both cups tend to get in the way of the sprayer’s view and both give a different balance to the gun. Feeding the spray gun with paint under pressure by hose from a remote tank avoids the work visibility problem, but is only worthwhile for the application of larger volumes of paint.
Since a significant amount of paint is lost in overspray, even with the more efficient HVLP spray guns, all air spraying must be done inside a spray booth. This is a legal requirement and the booth must conform to a national standard which will soon be the same as the new European standard. Please see our section on spraybooths. The spraybooth not only protects others from paint overspray and solvent vapours, but keeps dust away from the work being sprayed by filtering the incoming air.
The air supplied to the spraygun should be water and dust free as both will spoil the finish. It is normal practice to install particle filters in the air supply, usually upstream (the compressor side) of the air pressure regulator. These air filters may include a water trap that is drained by opening a tap from time to time, or automatic which is operated by a float valve. Whilst simple water traps may remove the bulk of the water, the air will still retain a certain amount of humidity and this can be removed by either a condensing dryer that cools the air down below the dewpoint, or by a chemical desiccant dryer. All bodyshop paint spraying must be done in a spraybooth that meets national standards; please see the section about spraying in a spray booth.