Siphon Feed or Regular Spray Guns

For many decades – since spray guns were first used to paint cars until very recently, in fact – the only spray guns available were siphon feed spray guns, mounting the paint cup beneath the nozzle and using air pressure to siphon the paint upwards into the paint needle valve, sucking it upwards by using an extremely high pressure – often between 30 psi and 50 psi for the older enamels and lacquers.

This high pressure was strong enough to make a vacuum in the siphon tube that connects the paint needle valve to the paint cup. The air is literally sucked out of the siphon tube, drawing the paint up after it, while air flows into the paint cup from another direction, using a vent port pierced through the top of the cup’s cover, on the upper surface of the cup. There is no force except the vacuum-creating passage of the air to move the paint out of the cup, so tremendous pressures are needed constantly in order to keep the spray gun supplied with paint.

Siphon feed spray guns are heavier and clumsier than HVLP spray guns, and the paint cup may rub or scrape against wet paint if you are not careful in how you handle the device. They also produce huge amounts of overspray and wastage because of the intense pressure at which the paint is blown out of the gun. This causes the droplets of paint to literally ricochet or bounce off the surface of the car – most of them are moving too fast to stick, and rebound from the panels like streams of tiny, liquid machine gun bullets bouncing off the armor of a tank.

The amount of wastage through overspray when using a siphon feed spray gun ranges from 65% to 75%, meaning that you may need to spray 4 gallons of paint for every one gallon that you need on the surface of your car. A two-gallon job could witness as many as six additional gallons literally vanishing into thin air, polluting the environment and multiplying the total cost of paint materials for the project fourfold.

There is no practical way around this with the siphon feed spray gun, since sufficient pressure must be maintained to keep the paint flowing out of the cup and into the air stream. Recall that typical automotive paint averages $300 per gallon at time of this writing, and the price of the overspray from a single car-painting job may become higher than the cost of the compressor and spray gun themselves. Siphon feed spray guns are also illegal in many places and the ban on their use is likely to continue spreading.

This type of spray gun has at least two advantages, however – one major and one minor. The major advantage to a siphon feed spray gun is that it can actually use a lower-strength compressor than an HVLP gun – although the high volume low pressure gun delivers paint at a nozzle speed of only 5 to 10 psi, around 60 psi is needed at the other end of the gun, at the hose inlet. Thus, a smaller compressor can be used with siphon feed spray guns. More minorly, siphon feed guns can be used more comfortably for spraying panels low down on the car.