Paint Guns - The Truth on Gun Control

Cleanliness
It is vital that you have a spotlessly clean (inside and, to a lesser degree, outside) gun that has a proper spray pattern. Spray guns are designed to atomize paint by mixing it with compressed air so it can be applied evenly and smoothly.

Operation and Controls
There are various types and shapes but they all work (basically) similar to a supercharged carburetor or mechanical fuel injection. That is, spray guns draw paint through a fluid tip where it is bombarded with precisely aimed compressed air from the air cap creating a venturi effect and a fine mist. The fluid needle controls the amount (via the trigger), with the paint supplied in a steady flow. In a suction gun (bottom cup), the material is drawn up by vacuum and in a gravity gun (top cup) by (you guessed it) gravity. Most guns have a fan control, which can narrow or widen the spray pattern. The other control is the amount of psi the incoming air is set to. Some guns have regulators built in or they can be added (with a small gauge) to the air fitting at the bottom of the handle.

Air Pressure
It is important that you use the correct air pressure for the material you are spraying and the type of gun. All paint manufacturers have tech sheets and/or info on the label as to recommended pressure with a conventional spray gun. For HVLP (High Volume Low Pressure) guns, follow the gun manufacturer's recommended inlet psi as these guns do not exceed 10 psi at the cap but require a lot more going in and vary greatly.

When spraying small parts, you can use less pressure as they will be easy to wet. If you use to much air pressure, you may create a lot of dust in your finish. If you do not have enough, the paint won’t atomize properly and you will get a heavy looking finish.

Test Patterm
Before you start painting, test your spray pattern on a scrap piece of paper. Start with the fan control wide open and spray some material at a normal distance (about 6 inches). You want a consistent and even line that is the same width with a small taper on each end. If you are getting a weird pattern (such as sideways) check your air cap under a bright light to make sure all the holes are clear and the end of the fluid tip that it seats on is clean in and out. Pull out the fluid needle and make sure it too is clean. If you are getting heavy spots at the top and bottom of your spray pattern (a sign of wear) adjust the fan control in just enough to even it out. If you can’t fix it and still have a wide pattern (8 –10 inches), you may need to replace the air cap, needle and fluid tip.

Cleanup and Pre-cleaning
Clean your gun with lacquer thinners. I usually pull apart the air cap, needle and fluid tip and soak them in clean thinners the day before I paint. Do not use metal wires to try and clean holes, use toothpicks or a gun cleaning brush as you can damage the precision of the jets.

Buying a Gun (no license required)
Like most things, paint guns vary in price and quality. For the casual user, a cheapie will probably suffice if you can get one with a small fluid tip. The cheaper HVLPs are generally awful at atomizing so stay away from them. Further, most cheap guns are set up to spray enamel where you want a lot of material coming out.

For base coats and clears, you need a smaller tip to force the paint to break up more with less material coming out. You can compensate for this a bit by turning in your fluid control so it restricts you pulling the trigger wide open (start at 1 ½ to 2 ½ turns in). Look for 1.5 mm in a suction gun and 1.3 mm in a gravity style. This will give you optimal atomization with just about any of the current urethane materials. If you are using acrylic enamel, a bigger tip will work fine.

As for suction versus gravity, it’s a matter of choice as they both have advantages and disadvantages when compared. Suction guns are heavier but hold more paint. Gravity guns won’t spray upside down but also won’t spit when you run low on paint. Suction guns don’t need a holder to fill them but are harder to clean. My own gun is a SataJet B gravity with a 1.3 mm tip (about 12 years old) although I also like the Sata 2000 HVLP also with the 1.3 mm tip (they run around $600.00).

John