Car Painting Tips: What You Should Know About The Spray Gun Pattern

Testing your Spray Gun’s Pattern

Indeed, test-spraying is an extremely fruitful way of detecting problems with either air or paint flow, and should be carried out before every major paint job. Some painters also shoot a test pattern periodically during a long, thorough paint job, using a piece of masked paper tacked up on the wall of their workspace as a testing surface. If you are painting an entire car, you should, at a minimum, test the spray pattern every time you fill the paint cup.

To test spray, set up your spray gun, filling the paint cup three-quarters of the way as usual, and connect the paint gun to the compressor. Set up a piece of clean paper or white cardboard in a vertical position, tacking it in place if necessary, at a height where you can spray it most conveniently – probably at about waist height, since holding your upper arm straight down at your side and your forearm out at right angles to your body is the most stable way to aim your paint gun.

Place the air cap around 8 inches (or 4 inches, if you are using a low-pressure gun, either high-volume or low-volume) from the paper’s surface. Then give a single rapid trigger-pull, opening the trigger fully and then immediately letting it snap shut, shooting a concentrated burst onto the paper or cardboard. Observe the spray pattern produced. If the spray pattern is a long, narrow, upright oval, then the spray gun’s parts are all properly adjusted and you can commence spraying.

However, you may encounter one of a number of different incorrect spray patterns. Each of these will reveal a distinct problem with the implement, requiring specific measures to correct it. Keep some brushes, solvents, and specialized air-gun lubricants on hand to deal with any problems, as well as a few wooden toothpicks (metal being too risky for reaming out air orifices) and whatever tools are necessary to disassemble your specific model of paint gun. Being prepared will save you much time in the long run.

Although handwriting analysis frequently fails to yield any useful results when applied to human beings – with only the most tortured explanations actually matching a person’s handwriting to the supposed personality traits allegedly revealed by this method – examining your spray gun’s “John Hancock” for revelatory information is much more straightforward.

Different Kinds of Incorrect Spray Patterns

•    If the spray pattern resembles the upper part of an exclamation point (without the dot – a rounded oval at one end, trailing off into a thinner point or even a sharp point at the other), oriented either way (point down or point up), then you have what is known as a top-heavy or bottom-heavy spray pattern.

To analyze, switch the air cap around by 180˚, and shoot another pattern onto the paper or cardboard. If the bulging end and the pointed end have changed places – that is, if a top-heavy spray pattern is now bottom-heavy, or vice versa – then the problem is some kind of debris in the wing ports. Clean the air cap in solvent and carefully ream out the wing ports with a wooden toothpick.

On the other hand, if the spray pattern does not change after rotating the air cap through a half-circle, part of the fluid nozzle hidden behind the air cap must be encrusted or filled with old paint, and requires cleaning.

•    If the spray pattern resembles a peanut or a barbell, with bulging ends and a constricted “waist” at the middle, then this is a split spray pattern. The first thing to check is air pressure, which might be too high – reduce the air pressure and shoot another pattern. If this does not correct the problem, then the nozzle is probably the wrong size for the paint viscosity, and a smaller nozzle opening is needed. Measure viscosity again, then place a correctly sized nozzle into the spray gun and shoot another test pattern.

•    If the spray pattern is an elongated diamond shape, with a bulging center and ends that taper away into sharp points, then this center-heavy pattern can indicate either excessively thick material is being sprayed, or the air pressure is insufficient. Try increasing air pressure, and if this does not work, either thin the paint or use a larger nozzle opening as appropriate.

•    A curved or crescent-shaped pattern is caused by one thing only -- one wing port is blocked and the other is open. You can determine which wing port is blocked by the direction of curvature – if the wing pattern curves to the left, then the left wing port is clogged, while a curve to the right indicates a congested right wing port.

If your spray pattern test is conducted and shows a correct, even, elongated oval without excessively pointy or ends or a noticeably bulging center, then all is well and you do not need to clean or adjust your spray equipment. However, any deviation from this “ideal” pattern requires you to work at bringing the spray pattern closer to the necessary benchmark, as follows --

Spray Pattern Width and Adjustment

If your gun is flinging a lot of paint onto the car’s surface, you can probably get away with a wider (and thus faster) pattern, while if volume is fairly low for whatever reason, then a narrower pattern is indicated. Use a tape measure to gauge both the distance from the air cap to the surface, and the width of the test spray pattern once you have shot it.

The testing sheet – which can be white cardboard, ordinary paper, or masking paper, since you will have this readily to hand in any case – should be mounted on a vertical surface, usually a wall, close to your work area, directly beside the vehicle to be painted. After all, you do not want to make an ideal spray pattern, and then be forced to disassemble, move, and reassemble the spray gun and its associated hoses at the worksite. This would defeat the whole purpose of adjusting the spray pattern to begin with, as the settings would not survive the transfer of the spray gun from one place to another.

To adjust the settings properly for a good coverage and spray pattern, the pattern should be tested at the painting distance. Keep the air open as far as it will go (within the pressure limits of the gun, of course) and then adjust the spray pattern using the material control to feed different amounts of paint into the air stream until you achieve the desired width of spray pattern.

4 to 6 inches is the usual width for a spray pattern, depending on the volume of material delivery; low volume calls for a 4” spray pattern, while 6” is more appropriate for high volume.  Six inch patterns are also good for pickup trucks, larger cars, and similar vehicles, while the smaller strokes are appropriate for some very compact cars, motorcycles, ATVs, and other lighter transportation devices.

Even if you have no problems with your spray pattern – such as a bulging center-heavy pattern, the peanut shape of a split pattern, or the cashew-like curve of a blocked wing port – then you will need to adjust the width of the pattern to match the materials you are using and the job you are undertaking.

Controlling the Spray Gun Pattern for Proper Painting

Since drenching the exterior of a car in a single, simultaneous, smoothly even coat of paint is impossible, it is necessary to paint a vehicle in swathes several inches wide, known technically as strokes. Strokes can be made either horizontally or vertically – horizontal strokes are much more common, and are used for most painting situations, but there are some occasions when a vertical stroke is necessary. The spray gun must be adjusted to each of these situations, which is fortunately a fairly simple operation.

In order to make a horizontal stroke along the panels of a car, a vertical spray pattern is needed – that is, the spray pattern must be tall and narrow. In order to obtain this kind of spray pattern, the air cap must be arranged so that the wing ports are projecting at the 3 o’clock and 9 o’clock positions, parallel to each other on the horizontal plane. In this way, their jets of air pinch in from the sides, squeezing the central spray of paint and air into a vertical lens and simultaneously helping to atomize the paint.

A horizontal spray pattern is needed for a vertical stroke along a car’s surface, with a wide, very short pattern – the exact opposite of the spray pattern for a horizontal stroke. This is achieved by turning the air cap until the wing ports project at 12 o’clock and 6 o’clock, paralleling each other on the vertical plane. Their air then pinches the spray from top and bottom, creating a broad but short lens of paint on the car surface.

Regardless of whether the contemplated stroke is horizontal or vertical, the distance of around 7 inches should be observed for high pressure spray guns, and less for low-pressure varieties. The reason for this is that being too close will “flood” the surface with liquid paint which will result in runs. When further from the surface than 7 inches, or at maximum 8”, the tiny, atomized droplets begin to merge in midair, becoming larger drops which spatter onto the sheet metal. This causes peculiarly knobby “orange peel” paint flaws – a hideous result, best to be avoided.
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