Setting Up a Spray Gun for Car Painting

This article covers the basics of setting up a spray gun for painting a car.

It is a bit like opening a safe with a combination: turning the individual dials of the lock is pointless until every element of the combination lines up, and only then does the door open.

Most spray guns have three main controls that let you dial in optimal atomization for different materials: fluid delivery, air supply and fan pattern adjustment.

How do the spray gun controls work?

The fluid control knob sits directly behind the needle. It is adjusted by turning clockwise (less paint) or counterclockwise (more paint). This changes the position of the needle. As the knob is unscrewed from the fully closed position, the needle moves back from the air cap, increasing the amount of paint ejected through the nozzle when the trigger is pulled.

The fan pattern control is usually located on the side of the gun body. It regulates the volume of air passing through the air holes in the horns of the air cap. The air coming out of these holes helps shape the paint stream and gives it a specific form. With the fan control fully open, air is distributed evenly through the air cap channels, creating an oval, cigar-shaped fan. Closing the control restricts the air passing through the holes, which changes the shape of the fan. Fully closing the control produces a round pattern.

Not all spray guns are fitted with an air adjustment valve. This valve lets you fine-tune the air supply after the required pressure has been set. It is usually recommended to keep the air valve fully open. Pressure is set with a gauge installed between the hose and the gun. The air valve controls the volume of air leaving the air cap holes, which helps break the paint up into the finest droplets.

How to set up a spray gun for painting a car

The main goal of the setup is to achieve a «cloud» effect when spraying, which shows that the paint is being properly broken up by the air.

In general, you should develop your own spray gun setup routine. Start with the basic recommendations and experiment gradually, adjusting the gun to your own preferences.

Regarding the air valve:

When the valve is screwed fully clockwise, the air supply is completely shut off. You can find the optimal air supply by trial and error. Too much air produces excessive aerosol particles, which leads to overspray and a dry paint film.

Too little air can mean the paint is not broken up into fine enough droplets. This shows up as large droplets on the surface when spraying. In that case the desired «cloud» effect cannot be achieved.

Many painters use a pressure regulator mounted at the air hose inlet of the spray gun. This is the most accurate way to achieve the recommended spraying pressure. If you use the regulator on the compressor, keep in mind that pressure is lost over every meter of air hose. This means the pressure set at the compressor will differ from the pressure actually at the gun.

Fluid control

Turning the fluid control knob clockwise reduces the amount of paint delivered.

To set this parameter, it is recommended to start with maximum fluid delivery and maximum fan width. To set the right pressure, pull the trigger partway so that only air comes out. Then, using the pressure regulator attached to the gun, set the desired pressure. After that you can pull the trigger fully to start spraying. Do a test spray on a surface (such as masking paper) to see how the paint is distributed. After changing the fan width, check the air pressure again, as it will change along with the fan. Then adjust the fluid delivery.

Too little fluid delivery can make painting the surface take too long. Too much fluid delivery can leave insufficient air to atomize the large volume of paint. When increasing fluid delivery, you should also increase the air supply and the speed at which you move the gun. Both of these parameters affect how evenly and how quickly the paint goes onto the surface.

Fan pattern control

This control lets you change the shape of the fan when spraying.

Some painters open the fan control all the way, but this can cause problems. It may lead to dry paint at the edges of the fan and slightly higher material consumption. So it is recommended to open the control fully and then turn it back a little.

A fan that is too narrow puts too much paint in one spot, which can cause runs and stripes. When you narrow the fan, reduce the fluid delivery as well.

The air cap can be rotated to change the orientation of the fan. A vertical fan is used most often, but a horizontal fan may be needed for painting narrow vertical parts.

The choice of setting depends on the specific job. For example, when priming a small area you need a narrow fan to avoid spreading primer unnecessarily. When painting a whole panel you need a wide fan to apply the paint evenly and avoid stripes.

Test spray procedure

Attach a piece of masking paper to a wall. This will be your spot for test sprays.

Set the required pressure at the gun using the regulator. Pressure should be adjusted with the trigger pulled. Start by closing the air supply completely, then slowly pull the trigger until only air comes out, without paint. Then set the desired pressure so that it stays stable while air is flowing continuously. After that you can release the trigger, and the gun will be set to the required pressure and air volume.

Next, turn the fan width control to maximum and then back it off slightly. Since changing the fan can affect the inlet pressure at the gun, correct the pressure after adjusting the fan.

Now close the fluid control completely. Then open it by 2 full turns (4 half-turns).

Begin the test spray.

Hold the gun squarely in front of the masking paper at a distance of 15–20 cm. Then pull the trigger fully and release it immediately. The test spray should form a «cigar» shape with even coverage inside and soft edges on the sides (see the illustration).

What problems can occur during the test spray, and what causes them?

The central area should be fully covered with paint without runs. If runs appear, the gun may be too close to the surface or the paint was sprayed for too long during the test. If the distance and spray time are normal, the fluid delivery is probably too high. Reduce the fluid delivery slightly.

The gun should spray tiny droplets while still giving good coverage density.

Excessive air pressure can produce extremely fine droplets that fly everywhere. At too high a pressure, material consumption increases. The paint that reaches the surface creates a dry, matte, dull finish. High spraying pressure can also create a figure-eight pattern, leaving stripes while painting.

Too low a pressure, on the other hand, produces large droplets, causing an «orange peel» effect on the surface.

Other factors that affect spray characteristics

There are additional factors that affect spray characteristics and do not depend on the gun settings.

Paint thinning

Thinned paint sprays more easily, but thinning also affects drying speed. Too much thinner can cause problems in the final result. As a rule, thinner should not exceed 10% of the paint volume.

Distance to the surface

A distance of about 30 cm from the surface is usually recommended. This value can vary depending on the viscosity of the paint. If the paint is too thin, it makes sense to increase the distance to the surface.

Gun travel speed

The speed at which the gun moves affects how much fluid is applied in a given time. Slow movement lays down a thicker coat. Fast movement gives a thinner coat. Moving too fast can result in insufficient paint, since the droplets will not flow together on the surface. However, all of this also depends on the gun settings and the paint viscosity. If the paint is heavily thinned and fluid delivery is at maximum, moving quickly while painting may be a necessity.