For aerospace painting, the air must reach the spray gun dry and free of oil and particles. In practice, that means at least a refrigerated dryer, which gives a pressure dew point of around +3 °C, oil aerosol filtration at the compressor outlet, then fine filters and an activated carbon filter at the foot of the spray booth. When the specification calls for a stricter ISO 8573-1 class for water, you add a desiccant dryer.
Why does air matter so much for aerospace painting?
An aerospace coating protects the structure against corrosion and has to withstand temperature swings, UV and vibration. It must last for years. An adhesion defect is not always visible when the part leaves the booth: it shows up later as blistering or flaking, and the whole area has to be redone.
Compressed air touches the surface at three moments: when blowing down before priming, when spraying the primer and when spraying the topcoat. At each stage it can bring three contaminants:
- Water, which condenses in the pipework and arrives as droplets or mist. It causes craters, blushing and adhesion defects.
- Oil, as aerosol or vapour. An invisible film on the panel is enough to make the paint bead up: this is the defect known as "fish eyes".
- Particles, rust from the pipework and dust drawn in by the compressor. They leave bits in the paint film.
Our article on paint defects caused by compressed air covers them in detail, with their causes.
What does ISO 8573-1 say?
ISO 8573-1 classifies air quality according to three contaminants, written in this order: particles, water, oil. The lower the number, the cleaner the air. For water, a few reference points:
| Water class | Pressure dew point | Equipment that achieves it |
|---|---|---|
| 4 | +3 °C or lower | refrigerated dryer |
| 3 | -20 °C or lower | desiccant dryer |
| 2 | -40 °C or lower | desiccant dryer |
For oil, class 1 limits total content to 0.01 mg/m³ and class 2 to 0.1 mg/m³. The prime contractor or the paint product's data sheet sets the class to meet, station by station. Our guide to ISO 8573-1 air quality explains how to read a class and at what point in the network it is measured.
Refrigerated dryer or desiccant dryer?
A refrigerated dryer cools the air to condense the water, then drains it away. It gives a pressure dew point of around +3 °C. As long as the air never drops below that temperature in the pipework, no more water condenses. That is enough for a spray booth in a heated building.
A desiccant dryer passes the air over a desiccant and gets down to -40 °C. You choose one when the specification requires it, when a pipe runs through a cold hangar in winter, or for certain primers and products that are sensitive to moisture. It uses part of the air to regenerate itself: include that in the compressor's flow. Our article on which air dryer to choose compares the two technologies, and the guide to pressure dew point explains the concept.
How do you remove oil from the air?
Our screw compressors are oil-lubricated: the oil cools and seals the screw, then is separated from the air in the separator. A small fraction is still carried over. It is captured by the filtration at the outlet, then by the filtration stages at the point of use:
- A pre-filter for particles and droplets.
- A coalescing fine filter for oil aerosols.
- An activated carbon filter for oil vapour and odours, just before the booth.
The fine filters are fitted at the foot of the booth, after the last branch to the workshop. Our article on oil in compressed air explains where it comes from and how to keep it down. One important point: there must be no lubricator on the paint line. Lubricated assembly tools have their own line.
What air flow does an aerospace spray booth need?
Take a small spray booth at a subcontractor painting structural parts and fairings: two painters work at the same time, one on primer and one on topcoat, with a blow gun for dusting off before painting.
- Topcoat spray gun: around 350 l/min, continuous.
- Primer spray gun: around 300 l/min, continuous.
- Blow gun: 150 l/min, in bursts, counted at half: 75 l/min.
- Subtotal: 350 + 300 + 75 = 725 l/min.
- 25% safety margin: 725 × 1.25 = 906 l/min.
- 20% expansion margin: 906 × 1.2 = 1,088 l/min, or 1.09 m³/min.
The K 75, at 1.1 m³/min, is right on the limit. If the workshop is also sanding at the same time, or if a desiccant dryer takes air for regeneration, choose the K 110 at 1.7 m³/min or the K 150 at 2.2 m³/min. These 4-in-1 compressors already include the refrigerated dryer and outlet filtration: all that remains is to complete the treatment at the point of use. Spray gun consumption varies from model to model, so the manual is the reference. See also compressor for an HVLP spray gun.
How do you prove air quality during an audit?
A prime contractor will not settle for a diagram. They want to see that the class is held over time. A few good practices:
- Keep a log of filter element changes, with the date and part number. Filters have a service life: a clogged or saturated filter no longer filters.
- Record the dew point at regular intervals if your dryer displays it, or have it measured.
- Have the class measured at the point of use by an independent body or by our technicians during a compressed air audit.
- Drain and check the automatic drains: a blocked drain sends water back into the network.
For every use of air in your trade, see our page on the compressor for aerospace. Send us the required air class and the list of your stations with a quote request, and we will define the treatment with you.
For your trade: Compressor for aerospace, aircraft maintenance and subcontracting.