Phoenix Journal · LEV Testing
Overspray drifting back at you is your spray booth losing its airflow balance - here is what causes it and how a measured LEV test puts it right.
Spray booth performance
If paint mist is drifting back at you, settling on the far wall or dusting the floor beyond the workpiece, your booth is telling you its airflow is no longer doing its job.
Overspray is the fine cloud of atomised paint that does not land on the panel. A little is unavoidable - even the best gun technique wastes some material - but a well set up booth pulls that cloud straight through the filters and away from the breathing zone. When you start seeing mist hang in the air, bounce back off the surface or coat things it should never reach, the issue is rarely the paint. It is almost always the air moving through the enclosure, and that is exactly what a proper examination is designed to check.
The good news is that oversp is a symptom you can read. Once you understand what balanced airflow should feel like, the causes narrow down quickly - and most of them are maintenance items you can stay ahead of rather than expensive faults.
A spray booth is a balancing act between air coming in and air going out. Fresh, filtered air is pushed in - usually through the ceiling in a downdraught booth - and a matched volume is drawn out through the exhaust filters. When those two volumes are in step, you get a smooth, even sweep of air that carries atomised paint away from you and traps it in the filter media. Overspray problems appear the moment that balance slips.
The single most common cause is loaded filters. As exhaust filters fill with captured paint, they resist the air trying to pass through them. The extract fan cannot pull the same volume, suction drops, and the mist that should have been whisked away instead lingers, swirls and drifts back. Meanwhile the intake keeps pushing the same volume in, so the booth becomes over-pressurised - and that excess pressure forces turbulent, uneven air movement that throws overspray around rather than carrying it in one clean direction.
Intake filters cause the mirror-image problem. When they clog, less clean air reaches the booth, suction from the exhaust wins, and you get patchy, weak coverage with dead spots where mist simply sits. Either way, the tell-tale is the same: paint going where it should not.
Notice that most of these are gradual. Nothing breaks with a bang - the booth simply drifts out of tune over weeks and months, which is why regular measurement matters far more than waiting for something obvious to go wrong.
The numbers that matter
You cannot judge airflow by eye. The reliable way is to measure air velocity across the working space with an anemometer, and UK guidance sets out what good looks like. These are the benchmarks a competent tester works to under the HSE guidance document HSG258, Controlling airborne contaminants at work.
Testers usually split the booth's open face into a three-by-three grid, take a reading in each square with a vane anemometer and average them. A booth that once comfortably held its mean velocity but now scrapes the floor of the range is a booth heading for overspray trouble - the falling numbers show up on the gauge long before the mist becomes obvious to the eye. That is the whole point of measuring: it turns a vague sense that something is off into a figure you can act on.
Start with the filters, because they solve most cases. Fit a pressure differential gauge - a manometer or Magnehelic gauge - across your filter banks if you do not have one already. Note the reading with clean filters as your baseline, and change the filters once resistance has climbed by roughly half an inch of water gauge above that figure. That rising pressure drop is the honest signal that filters are spent, and it arrives well before they look saturated. Relying on appearance alone almost always means changing too late, by which point overspray and contamination have already crept in.
Beyond filters, keep the extract ductwork clear of dried paint solids, check fan belts for wear and slippage, and make sure your intake and extract volumes stay matched so the booth neither over-pressurises nor over-extracts. It is also worth revisiting how the booth is set up in the first place, since the wrong specification will fight you forever - our guide to choosing the right spray booth for a body shop walks through matching a booth to the work you actually do.
Even a perfectly balanced booth cannot rescue poor gun discipline. High-volume, low-pressure guns, the correct tip for the material, the right distance from the panel and steady, overlapping passes all cut the amount of paint that becomes airborne mist in the first place. Less overspray at source means less for the booth to handle and cleaner results. Getting this right is as much a health matter as a finish one, because the same mist that spoils a job is what you must keep out of your lungs - the wider picture of spraying safely and staying compliant with booth and fume rules ties the two together. All of this sits under COSHH and, where flammable solvents are involved, DSEAR 2002, so keeping the booth in tune is a legal duty as much as a quality one.
The thread running through every cause and every fix is airflow you can prove rather than guess at. A drifting cloud of overspray is your booth losing that balance - and the surest way to catch it early, satisfy the 14-month examination requirement and keep both your finish and your people safe is a proper measured test by a competent engineer.
Questions
Loaded filters are by far the most frequent cause. As exhaust filters fill with captured paint they resist airflow, so the extract fan pulls a lower volume and mist lingers rather than being swept away. Because the intake keeps pushing the same volume in, the booth over-pressurises and the airflow turns turbulent, throwing overspray around instead of carrying it in one clean direction.
UK guidance under HSG258 points to a mean air velocity of around 0.4 m/s across the working opening, with no individual reading falling below 0.3 m/s. Where more toxic materials are sprayed, the velocity at the opening may need to be considerably higher - up to 1.5 m/s. A competent tester measures this with an anemometer across a grid of points and averages the results.
Under COSHH, local exhaust ventilation such as a spray booth must have a statutory thorough examination and test at least every 14 months. In practice many operators test more often, because the measured air velocity readings reveal a booth drifting out of balance long before overspray becomes obvious. Keeping to the schedule is both a legal duty and the best early warning you have.
The reliable method is a pressure differential gauge - a manometer or Magnehelic gauge - fitted across the filter banks. Record the reading with clean filters as your baseline, then change the filters once resistance climbs by roughly half an inch of water gauge above it. That rising pressure drop signals spent filters well before they look saturated, so you avoid the overspray and contamination that come from changing too late.
Phoenix Duct Clean · by the numbers
Phoenix examines and tests local exhaust ventilation to HSG258 and COSHH - measured, reported and certificated, UK-wide.