As the name implies, suction-feed guns use a siphon principle to deliver the paint to the sprayer by creating a vacuum in the paint reservoir that allows atmospheric pressure to push the paint into the line. Suction-feed guns use a small air compressor, except for the airless models that use a pump.
Suction-feed guns have the paint reservoir attached to the bottom of the sprayer handle that holds about a quart of paint. Because of the low reservoir volume, this type of paint spray gun is suited to small projects.
Gravity-feed paint guns have the reservoir attached to the top of the sprayer. Gravity-feed guns allow the weight of the paint to deliver itself to the sprayer.
Gravity-gun reservoirs are also limited to about a quart of paint and are also suited for small-sized applications. The main advantage of gravity guns is they don't require as much air pressure to operate, so they do not create an excessive amount of overspray, causing paint particles to fill the air and settle on nearby objects.
Pressure-feed paint sprayers work on the same principle as a garden hose spraying water. A container cable of holding very large quantities of paint is pressurized by an air compressor and delivers the paint to the sprayer through a long hose from the positive pressure created in the container.
Pressure-feed paint sprayers are used for large applications where a high volume of paint is needed, such as auto manufacturing plants.