Drying Waterborne Paints: Why DO You Really Need A Drying System?

Waterborne, waterborne, waterborne! It’s been pretty much all the auto refinish industry has been talking about for the last few years, and rightfully so. It’s a major shift. It involves new processes, new equipment, and of course, new environmental regulations. And, with any change like this, there arise many differing viewpoints on what changes are absolutely necessary, and what changes are simply ‘nice-to-have’s.

As far as paint booths are concerned, there are a few key factors that impact their performance with waterborne paints. The cleanliness of the booth is certainly a big one, since contamination particles are much more visible in a waterborne finish than in a solvent-based finish. Another is the need for super-clean, super-dry compressed air, since the slightest bit of oil or dirt in the lines can ruin your paint job. And of course, the big paint booth change that everyone talks about is some type of accelerated airflow system. But one aspect of this whole process that appears to be mostly overlooked is the actual science behind WHY those accelerated airflow systems are necessary. So let’s look at the way air typically moves over the surface of a vehicle in a paint booth.

Downdraft airflow is generally accepted as the best type of airflow for a paint booth, and generally speaking this is correct. It does an excellent job of controlling overspray and contamination, and provides a safe, clean environment in which to paint. However, there is one limitation that downdraft airflow just cannot avoid. This limitation is the creation of ‘laminar air’ and ‘boundary air’. Laminar air is created as air passes in one direction over a solid object in a paint booth. Boundary air is a low-pressure layer of slow moving air immediately below the laminar air and above the surface of the vehicle.

When looked at under a microscope, even the most perfect paint jobs are not entirely smooth. They have tiny bumps, dips and ridges that are inperceptible to the naked eye. These tiny imperfections slow down the air enough to create a layer of slow-moving air referred to as the ‘boundary air’. During the paint drying process, this boundary air becomes saturated with water molecules from the paint, and limits the speed of evaporation that can take place. It is this boundary air that prevents the airflow from drawing water molecules out of the wet paint.

In this video, we show how the air moves in a typical downdraft paint booth, with no accelerated airflow system active:

In order to achieve the fastest drying times possible, this boundary air must be disrupted and dispersed. This disruption is accomplished by creating turbulent airflow on the surface of the vehicle, which is what an accelerated airflow system does. It breaks up the slow-moving boundary air and rapidly speeds up the drying process.

Now, when an accelerated airflow system is introduced in to the equation, things are very different. By design, the most effective accelerated airflow systems are engineered to disrupt the laminar airflow by introducing air in to the booth that is moving at a different angle to the downdraft airflow. This is crucial, since the direction of the accelerated air will determine how effective the system is.

If an airflow system is simply designed to increase the top-down airflow over the vehicle, either through increased CFM through the air make-up unit or through a ceiling-mounted accelerator, the accelerated drying will be limited to being effective on the top surfaces (hood, roof, trunk) of the vehicle, and significantly less effective on the side surfaces (doors, quarter panels, bumpers). In essence, it compounds the laminar airflow problem and doesn’t provide the complete coverage needed to ensure that the entire vehicle benefits from the accelerated airflow.

However, if an accelerated airflow system is designed to produce air at a nearly right-angle to the downdraft airflow, the convection-effect created by this accelerated air will be much more prominent, and will produce the most effective drying environment and therefore the fastest drying times. The illustration at left provides a visual depiction of two counteracting airflow streams producing controlled turbulence on the painted surface, and how it uses that turbulence to draw vapors out of the coating much more rapidly.

In this video, we show how the air moves in the paint booth when the GFS AdvanceCure accelerated airflow system has been turned on:

The result of the powerful airflow is plain to see. The boundary layer is broken and the illustrative smoke is dispersed much more quickly. This rapid airflow allows the heated moving air to reach the painted surfaces, raise the skin temperature and draw the vapors and fumes out of the coating at a much faster rate. This minimizes the time required for flash-off and curing, and results in optimum curing for the best quality finish.

So, you can see the real reasons why an accelerated airflow system is so crucial. It completely changes the way air moves in the cabin of your booth during the flash-off and bake cycles. As mentioned earlier, different designs of accelerated airflow system will perform very differently, and it will be up to you to ultimately choose the system that best suits the goals of your shop. We hope that this information has helped to demystify some of the hype that is circulating out in the market, and that you have found it useful!

For more information, please visit www.globalfinishing.com.

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