Woodworker's Journal 2011-35-Winter, страница 64

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To achieve the results you want, move the gun evenly in a straight line (top). Resist the impulse to swing the gun up at the end of the board (bottom).

fairly low transfer efficiency at 25 percent or less and need a strong, efficient spray booth to keep overspray from becoming a problem. This is the least expensive of the three types of guns, and it is a good option if you own a compressor and want to spray several different types of materials.

HVLP Conversion Guns

Standard guns use fairly high air pressures but very little air. In contrast, HVLP guns use lots of air (high volume) at relatively low pressure: typically 10 psi (pounds per square inch) or less. The acronym HVLP stands for High Volume, Low Pressure. Conversion HVLP guns convert moderate volumes of high pressure air into low pressure air and should really be called MVLP guns.

These guns typically require a 5hp or larger compressor. They are tethered to it via a 3/8" air hose, and that means they are not really portable. Because you can increase the air pressure at the tip by turning up the pressure going into the gun, conversion guns are very versatile and can spray most any material, thick or thin.

They also boast a much better transfer efficiency at 35 percent to 65 percent — because they generate much less overspray. These guns are typically more expensive than their standard counterparts but may save you

money by wasting less finish. They're great for any type of finish, provided you own a large enough compressor.

HVLP Turbine Guns

Imagine a turbine blowing lots of warm air but at only five to ten psi. The turbine consists of a motor spinning from one to four fans, each inside a housing. The size of the motor controls how much air is moved (usually 80 cfin or more) and the number of fans, called "stages," determines how much pressure is generated. A one-stage turbine may create only five psi at the gun, while a three- or four-stage may get up to eight or even 10 psi. The gun itself is connected to the turbine by a rather large (1" diameter) hose. The turbine, hoses and gun are usually sold as a package and can easily cost upwards of $600. No compressor is needed.

These are lightweight, highly portable self-contained units that plug into a standard 110 volt outlet, making them ideal for on-site

spraying as well as shop work. They have a high transfer efficiency and may save money by reducing finish waste, but they lack versatility. There is no way to turn the pressure up, so you are limited by whatever the turbine produces. One that gives you only five or six psi at the gun may have problems spraying thicker finishes or even some water-based coatings.

Cup and Fluid Tip Options

Two important factors to consider in your choice of gun are the cup configuration and the range of fluid tips that are available.

Cups come in three different configurations. With a gravity feed cup, the finish flows down into the air path, thanks to gravity. That means all the air in the gun is used for atomizing the finish, not moving it out of the cup. This format is the easiest to clean and my personal favorite.

hi a siphon feed, air moving across the mouth of a small tube creates a vacuum, to siphon fluid up from the cup. Most HVLP guns do not have sufficient air pressure to power a siphon cup, but they are common on standard guns.

Finally, there's the pressure feed. Some of the moving air is diverted into a sealed cup, creating enough pressure to force the fluid up the tube to the nozzle. It takes only three or four pounds of pressure to move the finish.

The fluid tip is important because different types of finish work best with different sizes of tips. Lacquer and paint will spray

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With the adjustments wide open, the spray pattern should be oval and uniform (far right). If it is wetter on one side, pear-shaped, or bone-shaped (left to right), the fluid tip needs to be cleaned.

64 Understanding Spray Guns