Car Painting Tips: What You Should Know About Spray Guns?

Are Separate Spray Guns Necessary?


Although many auto painting gurus recommend this arrangement, others argue that there is no logical reason for it in most cases, since the paint or primer will need to be cleaned completely out of the gun in any case, and remnants of paint would probably crosslink and ruin the device before you got around to using the next color.


There are some circumstances where separate spray guns for primer and paint might be appropriate, of course. At a large auto painting shop, some painters may be undercoating cars, while those in adjacent booths are painting or clear-coating others.

In this case, if the primer has specific characteristics and needs – for example, if the nozzle size varies between the primer and paint, or if compressor calibrations are preset for the different materials in different booths– then the process might be streamlined by using separate guns for each purpose. The whole “set-up” can be left in place, and the car taken to the appropriate gun for the layer of surfacing material that is about to be applied.

On the other hand, for smaller operations – including individual hobbyists or part-time car painters – keeping a separate spray gun for each material could prove to be a needless expense. You can only use a single gun at a time in this instance; the gun will need to be cleaned thoroughly after each use regardless, so that it will continue to function, so there is no chance of cross-contamination between materials; and since your projects are more infrequent, changing the nozzles and compressor settings is an occasional task that will not be much of a hardship.

If you need more flexibility for applying small areas of paint, or reaching inaccessible spots, then buying a touchup gun, or jamb gun as it is also called, is a good option for your workshop. This gun will be much cheaper than a second full-size spray gun and will add to the versatility of your painting in any case – a perfect combination for smaller painting operations.

The cost of a good spray gun is not negligible – at around $350 to $400 apiece, they cost as much as lower end personal or notebook computers – so the question naturally arises whether it is necessary to buy a spray gun for primers and another for paint and finishes.

The Functioning of a Spray Gun

Setting up and using a spray gun involves basic knowledge of how the device works, so that it is possible to judge when it is working correctly, and to determine what kind of air cap and other accessories you need. One thing that you must determine is what air cap is necessary for spraying the type of paint you are planning to use to coat your car. If the central orifice is too small for the thickness of paint you are using, it will clog – like trying to force toothpaste through a pinhole – while if it is too large, the spray pattern will be too loose and thin to create a good coating on the metal.

You can learn the size of air cap orifice needed by using a viscosity cup. These cups are small metal or plastic containers with an almost funnel-like design. Fill the cup with the paint, primer, clear coat, or whatever you are planning to use, then time the period it takes for the substance to drain out of the cup through its bottom hole.

Compare this to the chart that was supplied with the viscosity cup and you will know the viscosity of the paint. Note that viscosity is only measureable with any accuracy while the temperature of the paint or other substance is between 65˚ and 75˚ Fahrenheit. Heat above 75˚ F will make the paint a bit runnier than it would be at ordinary room temperature, while lower readings will make the paint sluggish and result in an inaccurately high viscosity reading. Once you have determined the viscosity, you will be able to find the type of air cap you need, possibly in consultation with paint store personnel.

The wing ports or horn ports – those openings on each side of the air cap which feature their own diagonally-aimed air orifices – send a converging blast of air towards the jet of air and aerosol paint that is being projected from the spray gun’s main orifice when you are shooting. Without these wing ports, the spray would jet out in all directions, forming a broad cone and producing only fitful, unpredictable coverage.

However, the air from the wing ports compresses the spray into a tall, flattened, vertical oval which paints a stripe of the vehicle’s surface thoroughly and evenly, producing the high-quality results needed for car painting. All of the orifices must be kept clean to keep the spray pattern correct.

Spray pattern might appear to be an unimportant detail to the layman, but in fact, any distortion will lead to irregularities in the thickness of the paint on the car’s surface. Too thick paint may run or even crack as it dries, while too-thin paint will be vulnerable over time. It behooves you to periodically test your spray gun’s spray pattern before making use of the device, to ensure that it is still working properly.

Protecting Yourself while Spray Painting




A regular half-mask respirator, even one with a robust filter, is not enough protection for working with modern urethane paints because of the isocyanates they contain. These potentially lethal chemicals can soak into the eyes as well as the lungs when airborne, and cause sickness or even death. Any mucous membrane will do, making them more than simply a respiratory hazard. Work is being undertaken to produce less dangerous paints, but it is best not risk exposure to the current products.

The surest way to protect against them is to wear a fresh air hood, which is hooked up to a small compressor in another room where clean air is available, or even outdoors upwind of the ventilation outlets. This blows pressurized air over your face behind a clear plastic face shield, thus giving you good air to breathe and keeping the paint droplets out of your eyes.

You should also wear a disposable painter’s suit, which keeps irritating paint off your skin and prevents garment debris from falling onto the paint surface, creating flaws. Rubber gloves are also a necessity, and the cuffs of all garments should be taped to keep paint out of your clothes.

Finally, you must ensure that there are no sources of flame or electrical sparks in the workspace, since paint vapor is very flammable and a pilot light or the starter of an electric pump motor could potentially trigger an explosion. The more paint vapor there is in the air the more unstable it becomes, until at a certain point, it will detonate with the smallest friction as a trigger. Making use of all these safety precautions is the best way to ensure that you will still be in a condition to finish your project by the end of it – and go on to create more notable paint jobs in the future.

Although ensuring that your workspace has proper ventilation will help to generally increase your safety and produce a better finish on your car, you also need to take care to directly shield your body. This will also protect the work from coming into contact with the natural skin oils of your hands, or the lint and dust from your clothing.

Cleaning up a Spray Gun after Use

Paints and all types of paint-related products – including primers, clear coats, and so on – harden eventually, and leaving even fairly limited traces of any of these substances in your spray gun after you have used it is an invitation to reducing your favorite painting implement into a useless and more or less solid lump. Even if the dried paint can eventually be soaked out with solvent, a great amount of time and effort will be wasted performing a cleanup which could have been avoided by clearing the gun out initially immediately after use.

The first step in the process is to acquire the solvent that is offered as part of the paint system you are using. All complete paint systems should include plain, compatible solvent, both to further thin paints if necessary and, more commonly, for cleaning tasks such as this. These solvents should be bought at the same time as the paint, both to avoid any problems caused by reformulations occurring before you get the chance to use the paint, and to ensure that you do not begin spraying paint and suddenly discover that you have no available way to clean your spray gun.

Further techniques to clean up your spray gun after use include –
  • Remove the paint cup, slosh some solvent around inside to carry out an initial cleaning, then fill the cup halfway with new solvent and spray it out of the gun exactly like you were applying paint. Repeat these steps several times until the solvent coming out of the air cap is clean and untinted, showing that most of the paint has been removed from the depths of the device.
  • Remember that polyester primers are extremely resilient and harden in 25 minutes – probably beyond any hope of recovery. It is necessary to work fast when these products are involved.
  • Some gravity-feed guns include disposable plastic liners for the paint cup. These are probably not the most environmentally sound items, so some painters may wish to dispense with them to avoid creating more pollution than the painting process already does. However, many will prefer the simplicity these liners add to cleaning, since there will be no rinsing of the cup necessary and you can go straight to spray-cleaning the gun.
  • Once this has been done, take off the air cap and place it into a shallow tin of solvent, along with its associated retaining ring. The paint needle valve must be extracted by loosening the material control until it comes free, after which is should be washed in solvent and sparingly anointed with special paint gun lubricant at the places where the trigger contacts it.
  • The interior can now be cleaned out with a pipe cleaner or slim brush, and the air cap ports opened with a wooden toothpick and some solvent. Do not use metal on the gun while cleaning it, since the gun’s performance depends on precise tolerances and even a slight scratch might result in drastic changes in the painting results.
  • Even the humble toothbrush has a role to play in cleaning a spray gun. If paint got on the exterior, it should be wiped down with solvent and a clean cloth, with a toothbrush or similar small but relatively soft brush used for the more stubborn places.

Safety Considerations while Spray Painting

It is often the invisible perils of our world which are the most dangerous – since they are the most difficult to avoid. Many a stout swordsman of the old European warrior-culture was brought low not by the skillfully-wielded weapon of an enemy, but by legions of bacteria lurking unseen in drinking water that was not properly boiled. Many a doughty laborer and leathery explorer have been felled by the microscopically small pathogens in the beak of a mosquito or a flea. In the modern age, these microbial dangers are somewhat better controlled and contained, but there are many hazards just as insidious and unseen, though of different character – springing, this time, from chemistry.

The overspray accompanying even the cleanest spray-gun’s action is one of these risks, so it is important to protect yourself in several ways when painting a car. The process of painting – even when it uses LVLP (low volume low pressure) equipment – creates clouds of overspray which may contain anywhere from 30% to 75% of the paint that you are blasting onto the car’s surface. The air pressure drives the paint droplets so hard that many of them bounce off the sheet metal rather than adhering, and form a billowing mist around the painter and the car alike.

Ventilation is the first safety precaution that is needed, because it helps with handling the other precautions that are needed in the car painting environment. Many kinds of paint need to be sprayed indoors because of the risk of surface contamination, and indoor spaces are perfect for trapping and building up fumes to dangerous levels.

Whether you are using a professional spray booth or your garage, you need both inlets and outlets which will move a sufficient volume of air to keep up with the overspray. The inlets should be at one end of the garage or workspace, and the outlets at the other, to keep an even, steady, predictable air flow moving through the space. This allows you to start spraying the car at the end nearest the inlets and work forward towards the outlets, thus ensuring that overspray droplets will fall on unpainted areas rather than already-painted ones.
References:
How To Spray Paint Your Car - Auto Painting & Bodywork Repair
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