When craters, bubbles or water spots appear in a paint finish, compressed air is often to blame, but not always. A test of a few minutes, with an empty gun, settles the question. If the air comes out wet or oily, the fix follows an order: draining, dryer, filtration at the station, then the condition of the compressor.
Defects that can come from the air
| Defect | What you see | Possible cause in the air | Other causes to rule out |
|---|---|---|---|
| Craters, fish eyes | small round dips where the paint pulls back | oil aerosols, water droplets | silicone, polish, poor degreasing |
| Bubbles, pinholes | small holes or raised spots in the film | moisture in the atomising air | coat too thick, wrong thinner, temperature |
| Water spots | light marks or local runs | liquid water condensed in the hose | |
| Dirt, grit | hard inclusions in the film | particles, rust from the pipework | booth filters, clothing, sanding nearby |
| Uneven colour | finish or shade that varies from panel to panel | pressure dropping at the gun | gun settings, spraying distance |
A workshop that paints and sands on the same network often combines several of these causes. Our page on compressors for body shops presents the starting installation. Here we look for what is wrong in an existing one.
The test, with an empty gun
- Empty and clean the gun, or use a clean blow gun on the same outlet as the spray gun.
- Blow for about thirty seconds onto a clean mirror or a sheet of white paper, a few centimetres away.
- Look. Mist that stays or droplets: water. A greasy film or spots that do not evaporate: oil. Dark specks: particles or rust.
- Repeat the test at the dryer outlet, then at each outlet up to the station. If the air is clean at the compressor and dirty at the station, the problem is in the pipework.
If the air stays clean at every point, look at the substrate, the product or the booth.
Where does the water come from?
Intake air contains water vapour. As it cools after compression, it condenses in the receiver and pipes, and ends up at the gun if nothing stops it. The most common causes:
- no dryer, or a dryer bypassed by its bypass valve;
- a dryer that dries poorly: dirty condenser, room too hot, dryer undersized for the compressor;
- a receiver never drained or a drain stuck closed;
- a pipe that runs through a cold area after the dryer, where water condenses again.
Our article on water in compressed air details the drain points. An automatic condensate drain makes sure none is forgotten.
Where does the oil come from?
Our compressors are lubricated. On a screw compressor in good condition, the air-oil separator holds back most of the oil, and filtration stops the remaining aerosols. Oil reaches the gun when one of these barriers stops working:
- a worn air-oil separator, to be replaced at the hours given in the manual. See our article on the air-oil separator;
- a saturated filter element: a clogged filter no longer filters;
- a piston compressor running continuously, which heats up and sends more oil into the network;
- a tool lubricator fitted upstream of the paint station.
The fixes, in order
- Drain the receiver, the dryer and every filter. Fit an automatic drain wherever draining is manual.
- Check the dryer: running, clean condenser, bypass valve closed.
- Replace filter elements at the scheduled intervals.
- Add fine filters at the paint station, after the last cold point in the network. Our article on fine filters explains the order of fitting.
- Separate the paint line from the tool line, and remove any lubricator from the paint line.
- Service the compressor: first service at 500 hours, then every 1,500 hours on our screw compressors, with the model's service kit.
- Replace an overloaded piston compressor with a screw compressor if sanding and painting run all day.
Starting again on a sound installation
- The K 75: 4-in-1 screw compressor, 1.1 m³/min at 8 bar, 420 L receiver, built-in refrigerated dryer and filtration. The air leaves already treated.
- The K 110: 1.7 m³/min at 8 bar, 340 L receiver, same air treatment, for a workshop that paints and sands at the same time.
- The S 110 with a refrigerated dryer sized for 11 kW: when the workshop already has its receiver and wants to keep its pipework.
In every case, the air class at the paint station is set with our technicians according to ISO 8573-1, taking the paint into account. For a full check of the network, our compressed air audit looks at air quality, pressure and leaks, from the compressor to the station.
For a dedicated booth, see our page on compressors for spray booths. Describe your defects and installation in a quote request: we will look for the cause with you.
For your trade: Compressor for body shops.