A spray booth counts the dust in its air, because dust in a wet coat is a job done twice, and the count has to be near zero before the gun comes out. Ours reads the dust of somebody sanding filler, at sanding levels, in a booth that is sealed, unlit and empty, most mornings before anybody has clocked in.
I run the bodyshop at a dealership, and the booth is the room the whole business turns on.
How the counter works.
It draws air through a laser and counts the particles that cross the beam by size. The booth runs its fans to purge before a job, and the count comes down to a few hundred per litre, and then the painter goes in. A count in the tens of thousands means somebody is sanding, or the filters are gone, and you do not paint.
The counter logs every minute, and I read the log because the warranty on the paint depends on the booth having been clean when it was applied.
The booth reads forty eight thousand at six in the morning, most mornings, with the doors sealed, the fans off and the lights out.
Forty eight thousand is a person sanding filler with a block, in the booth, about a metre from the counter. I know because I have sanded filler a metre from the counter and read it.
The count starts to rise at about half past five, climbs to that level over ten minutes, holds for forty minutes with the small variations of somebody working, and then drops over half an hour to the background, which is what the count does when the sanding stops and the dust settles.
Nobody clocks in until seven. The booth has a door contact and it does not open. The bodyshop is alarmed and the alarm is set.
What has been checked.
The counter. Serviced, recalibrated with a reference aerosol, then replaced with the newer model, which also counts by size and which says the particles are in the size range of sanding dust and not of the fine stuff the filters let through.
The booth. Sealed and smoke tested. It holds pressure. There is no way in for dust except through the filters, and the fans are off, so nothing is moving air through them.
The dust itself, which I did on my own time. I put a clean sheet of black card on the booth floor overnight, by the counter, and in the morning it had dust on it.
Filler dust. The grey powder that comes off a block on cured filler, fine and dry, in a light drift, more towards the middle of the booth than at the edges, the way it lands round somebody working.
I have had it looked at by the paint supplier's lab. It is polyester filler, our brand, sanded.
Where the job is.
There is nothing in the booth. That is the fact I keep returning to. A car in the booth, with filler on it, sanded by nobody, would be strange. An empty booth, with the dust of a car being sanded landing on the floor where the car would be, is stranger.
I have marked the floor. The dust falls in a shape, and the shape is about the length and width of a car, and it is in the position a car sits when it is in for a rear quarter, which is the job that makes the most dust and the one my best painter, who died in 2019, did more of than anybody, because he was the best at it.
What is done about it.
The booth is purged at seven, the count comes down, and the day's work goes in clean. The log shows a spike before opening that I have annotated as pre-shift contamination, and the paint supplier's warranty auditor has seen it and said the purge was adequate.
I have not told the painters. Two of them worked with him.
What I do.
I read the log and I purge the booth.
The one thing I have changed is that I do not go in before seven.
I used to do the morning check at six, in the booth, with the lights on, and I did it for years.
In 2022 I went in at ten to six because the counter was already climbing and I wanted to see the air, and I put the lights on, and the booth was empty, and the air in the beam of the lights had dust in it, hanging and drifting the way sanding dust does, in a cloud about the height of a man's shoulders, over the place where the rear quarter of a car would be, and it was moving in short strokes, back and forth, about the length of a sanding block.