Specifying a Spray Booth Correctly

What a correct specification actually covers

Buyers tend to fixate on the shell - the size, the doors, the lights, the finish. Those matter, but they are the easy part. A proper specification is really a set of performance commitments, and the shell is just the box that delivers them. In practice you are pinning down six things at once: the booth type and airflow pattern, the capture velocity across the working zone, the heating and air-replacement arrangement, the filtration, the fire and explosion controls under DSEAR, and the way the whole thing will be tested and maintained once it is running.

The governing standard in the UK is BS EN 16985:2018, "Spray booths for organic coating material - safety requirements". It covers both liquid and powder coating and deals with the significant hazards across design, installation, operation and maintenance. If a supplier cannot tell you how their booth meets it, that is your answer. Sitting alongside it are the HSE guidance documents you will be measured against in practice - HSG258 for the design and testing of LEV, HSG276 for the wider workshop, and HSG140 for the safe handling of the flammable liquids you will be spraying.

Airflow is the number that matters most

Everything downstream depends on air. HSE guidance for spray booths asks for an average air velocity across the sprayer's breathing zone of at least 0.4 metres per second, with no single measured value falling below 0.3 m/s. That is not a comfort figure - it is what keeps the concentration of solvent and isocyanate below the workplace exposure limit and clears the mist quickly enough to work safely. Under-specify the fan and make-up air and you can never reach it, no matter how good the operator is.

Airflow is delivered by the booth type, and the three common patterns behave very differently. A cross-draught booth moves air horizontally from the front to an extract at the rear; it is the cheapest to buy and run because it needs smaller fans, less make-up air and fewer filters, but it drags overspray across the work and struggles on larger items. A semi-downdraught pulls air down over the vehicle and out at low level to the rear - a sensible middle ground. A full downdraught draws clean air from the ceiling straight down to a filtered pit floor, which gives the most even flow and the best finish, and is what most quality bodyshops and industrial finishers specify. It is also the most demanding to feed with heated make-up air, which is exactly why it has to be specified deliberately rather than assumed.

Match the booth to the work, not the brochure

The most expensive specification mistakes are the ones that look fine on paper. A booth sized for saloon cars will choke on light commercials. A cross-draught chosen to save money will fail a bodyshop that sprays large panels in metallic base coats, where uneven airflow shows up instantly in the finish. And a booth ordered without a heated air-replacement unit will never hold a bake cycle, so cure times stretch, throughput collapses and the numbers that justified the purchase never arrive. If you are still weighing the options, our guide on how to choose a spray booth for a body shop works through the trade-offs in plain terms.

Filtration deserves the same care. Intake filters keep dust off the wet finish; extract filters strip overspray from the exhaust so you are not discharging paint solids to atmosphere or blinding your fan. Good multi-stage extract media capture the great majority of overspray, but they only work if the booth is specified to hold the right filter grade and if someone changes them on a schedule. A booth that is overspraying is very often a booth whose filters are loaded, whose balance between intake and extract has drifted, or whose airflow was marginal from day one - the causes we set out in why your spray booth is overspraying.

Then there is the part of the specification that keeps people alive rather than merely productive. Because you are atomising flammable solvent, the booth interior is treated under the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) as a potentially explosive atmosphere. The cabin is typically classified as an ATEX Zone 1, with a hazardous zone extending outward from the working opening, so any electrical equipment, lighting and lift trucks operating in or near it must be suitably rated. Interlocks that stop spraying if the fan fails, correct earthing to prevent static discharge, and non-sparking construction are not optional extras - they are load-bearing parts of the specification, and a competent examiner will look for them.

Finally, specify the bake or cure arrangement around the actual coatings you use. A heated make-up air unit that lifts booth temperature to a controlled bake - commonly around 60°C for automotive refinish - transforms throughput, but it also lengthens the clearance time before anyone can safely re-enter without breathing protection. HSE publishes guidance on measuring those clearance times precisely because getting them wrong exposes sprayers to isocyanate mist they cannot see. Build the airflow, the heating and the clearance regime into one coherent specification and they reinforce each other; treat them as separate line items and they fight.