A spray gun atomises paint, lacquer or glaze by passing a fast jet of compressed air over a liquid so the air draws the liquid up and breaks it into droplets, and it is the hungriest air tool in the shop: a 2 mm air jet at 3-4 bar gauge can pass up to 148-185 L/min of free air. Spraying a solvent-based finish also needs a ventilated space with no flame or spark, and a respirator, because the mist is flammable and harmful to breathe.
Part of Pneumatic Systems
A spray gun is a nozzle with no moving parts. Air rushing past a liquid feed tube draws liquid up and shatters it into a fine mist that coats a surface evenly.
The Air Jet Does the Work
At its simplest, per the carrier, the gun is a Venturi nozzle that draws liquid from a reservoir and atomises it with compressed air; a 2 mm nozzle at 3-4 bar produces a fine spray suitable for paint, lacquer and glaze. The jet is fast because the air expands from gauge pressure to atmospheric pressure through a small hole. Fast air has low pressure beside it, so the liquid is pushed up the feed tube by the atmosphere and torn into droplets at the tip.
It Consumes a Lot of Air
Treat the carrier’s 2 mm as the air orifice. Once the pressure ratio is above about 1.9 the flow is choked and depends only on pressure and area. Using the choked-flow formula (recalled from compressible-flow texts):
- Orifice area = π / 4 x 0.002² = 3.14 x 10⁻⁶ m²
- At 4 bar gauge (5 bar absolute) the ideal mass flux is about 1,196 kg/(m² s), so the mass flow is 3.76 x 10⁻³ kg/s
- Divided by the density of free air, 1.204 kg/m³, that is 3.12 x 10⁻³ m³/s, 185 L/min
- At 3 bar gauge the flow scales with absolute pressure, 4 / 5 of that: 148 L/min
These are upper limits, because a real nozzle passes less than the ideal. Even so, a compressor of the carrier’s reference size delivers 235.6 L/min before losses (Piston Compressor), so one spray gun can consume most of it. A receiver (Air Storage and Treatment) carries a short spraying job but a long one needs the compressor to keep up.
Clean, Dry Air Is Part of the Equipment
Water and oil in the air spit onto the surface and ruin a finish. Fit a water trap and a drain on the line and drain the receiver before spraying; see Moisture Removal. Strain the paint, thin it as the product requires, and keep the gun clean after each use.
Common Mistakes
- Spraying from a hose that has pressure drop. A small bore hose starves the gun; see Pipe Sizing.
- Too much pressure. It throws more of the paint into a mist that never lands on the work, wastes air and raises the cloud you must breathe.
- Spraying where the mist can reach a pilot light, forge or stove.
- Pressurising a paint container beyond its rating. A feed vessel is a pressure vessel; a can or jar of unknown strength can burst.
Safety
Solvent mist is flammable, harmful and hard to see
A cloud of atomised solvent-based paint or lacquer can ignite from a flame, a spark, a hot surface or a static discharge. Spray outdoors or with strong through-ventilation, put out every flame and keep every spark source away, including the forge, the stove and the grinder, and never test the air for vapour with a flame. Wear a respirator that suits solvent vapour and mist; the corpus’s Dust and Respiratory Protection covers dust, not solvents, so it does not settle the choice. Keep the quantity of solvent small and closed, wear eye protection, and never spray towards another person. Pesticides and other poisons need their own protection and handling; do not spray them without the protective equipment the product requires.
What’s Next
- Learn next: Blow Molding Basics — another job for a steady, clean air supply
- Combine with: Pneumatic Hammer — a second air tool that sets the compressor’s size