- A spray booth pulls mist away from the sprayer, filters it and sends it outside. It runs at lower pressure than the room around it so nothing leaks out.
- A pop-up tent or inflatable booth keeps dust and insects off wet paint. It does not protect your lungs.
- Inflatable booths blow air in. That is the opposite of how HSE says a booth must work.
- Spray mist is invisible in normal light and hangs in the air for minutes after you stop.
- Solvent vapour burns. Remove every source of ignition before you spray indoors.
What a booth does
Professional restorers and bodyshops spray inside a booth because of what HSE found when it lit the mist properly. Its leaflet on isocyanate spraying says a gun "creates a visible fan of paint, and large quantities of paint mist that is invisible under normal lighting", and that "the mist quickly spreads through the whole spray enclosure, enveloping the operator."
The ventilation doesn't whisk it away, either. HSE again: "The paint mist is not, as is often imagined, instantly removed by the ventilation. The ventilation air is overwhelmed by the spray-gun air jet and the invisible mist builds up."
So a booth does three jobs. It moves a large volume of air past the work in one direction. It filters the mist out of that air and discharges it outside. And it runs "at a slightly lower air pressure than the surroundings", which HSE calls negative pressure, "to prevent paint mist escaping into the workplace". A gauge on the wall proves it, and the whole system is examined and tested at least every 14 months.
Every booth also has a clearance time, the minutes it takes to clear the mist after spraying stops. HSE says a booth typically clears in under five minutes and a spray room can take twenty or more. Nobody takes their breathing apparatus off until that time is up.
What a tent does
A pop-up spray tent is a fabric box with a floor and a mesh door. It stops dust, leaves and insects landing in wet paint, and it stops overspray settling on the car next to it. Those are real benefits, and for one-pack paint outdoors or in a draughty garage a tent improves the finish.
It has no extraction, no filtration worth the name and no negative pressure. The mist stays inside with you, more concentrated than it would be in the open air. Wear your mask inside a tent without fail, and leave the door open to the breeze.
Inflatable booths look more convincing because they come with blowers and filter panels. Look at which way the air goes. One blower holds the structure up and another pushes filtered air into the working space, which leaves through panels at the far end. The inside is at higher pressure than the outside. That keeps dust out well, which is what the makers sell them for. It is the reverse of the negative pressure HSE requires, the exhaust air is not ducted to a safe place, and nobody has measured a clearance time. For two-pack paint an inflatable booth is not a spray booth, and neither we nor HSE would treat it as one.
Spraying one-pack paint at home
Most home engine painting uses one-pack enamel, which contains no isocyanate. The mist and solvent are still harmful, as the safety page sets out, so the aim is to keep both away from your face and out of the house.
Outdoors on a still, dry day is the simplest answer for small parts. A gentle breeze at your back carries the mist away from you. Wind carries dust into the paint and the paint onto the neighbour's car.
In a garage, open the main door and a window or door at the other end so air moves through in one direction, and stand upwind of the work. A fan can help if it blows clean air in from behind you. Don't put an ordinary fan in the path of the solvent-laden air going out, because its motor can spark.
Give the mist time. HSE says even an extracted spray room can take twenty minutes or more to clear, and a garage has no extraction at all. Keep the mask on until you have left the space, and stay out at least that long before going back without one.
Fire
Aerosol engine paints are extremely flammable. Hycote's data sheet gives a flash point below minus 20°C, which means the vapour will ignite at any temperature you will ever work in.
Before spraying indoors, turn off anything with a pilot light or an element: boilers, gas heaters, fan heaters, halogen lamps. Don't spray near a running compressor motor, battery charger or freezer. Keep a fire extinguisher by the door you would leave through. HSE's guidance for workshops adds a sensible limit: don't store more than 50 litres of flammable liquid indoors.
Used rags and abrasive paper soaked in paint or thinners can heat up and catch fire by themselves. VHT's safety sheet says to put them in a sealed metal container filled with water.
If you want two-pack
Then you need a real booth and air-fed breathing apparatus, and the practical route is to take the parts to a bodyshop that has both. A block and a set of covers is a small job for them. The two-pack page explains what the paint is, and the masks guide explains what the breathing apparatus is.
See the spray shelters in the shop.
Sources
- HSE INDG388: safety in isocyanate paint spraying: invisible mist, clearance time, negative pressure, 14-month test
- HSE: measuring clearance times in spray booths: how clearance time is measured and displayed
- HSE MR1: mixing two-pack paint: ventilation discharged to a safe place, the 50 litre storage limit
- Hycote Engine Enamels technical data sheet: extremely flammable, flash point below minus 20°C
- VHT Hi-Temp Engine Enamel safety data sheet: rags and residue can ignite spontaneously