With the same paint, the same spray gun and the same settings, the finish is even one day and has an orange peel appearance the next. The difference has several possible causes, including paint viscosity, the application method and the conditions in the paint booth. Air humidity is one of the factors that should be checked when the result changes.
In the automotive industry, the finish must have the same quality from one car body to the next. That is why stable booth conditions matter, alongside the spray gun settings. Condair, a manufacturer of humidity control equipment in the Texco portfolio, describes the case of a car plant where the share of car bodies accepted at the first inspection after painting increased by 10% after humidity control was improved.
With waterborne paints, warm and dry air speeds up evaporation. The paint loses water during spraying and after it reaches the part, and the coat has less time to flow out evenly. This article explains what changes with humidity and how the right conditions are set for a consistent result.
What happens to the paint between the spray gun and the part?
The spray gun atomises the paint into fine droplets that travel towards the part. Evaporation begins along this path and continues after the paint is deposited on the surface.
The rate of evaporation depends on air temperature and humidity, as well as on air movement. In warm, dry and well-ventilated air, the water in waterborne paints evaporates faster. In humid air, evaporation is slower.
Waterborne paints are particularly sensitive to these variations, because air humidity directly affects the evaporation of the water they contain. Booth conditions also matter for solvent-based paints, including through their influence on static electricity and dust deposition. The effect on the finish depends on the paint formulation and the application method.
What humidity is required in paint booths?
In some processes that use waterborne paints, a relative humidity of 65-75% is maintained. This range helps control evaporation during spraying. The conditions required for each process are set according to the material’s technical data sheet.
Primer, basecoat and clearcoat may have different requirements, even when they are applied on the same line.
The humidity value is interpreted together with 2 other factors:
- Air temperature. Relative humidity depends on temperature. If the air is heated without receiving additional water vapour, the percentage shown by the sensor drops, even though the amount of water in the air remains roughly the same.
- Air movement. The air supplied to the booth and the exhaust air influence evaporation. Two booths with the same temperature and the same relative humidity can have different drying times if the air moves differently around the parts.
What happens when the air is too dry or too humid?
Air that is too dry
With waterborne paints, rapid evaporation reduces the time the coat has to flow out on the part. Some droplets lose too much water before reaching the surface and contribute to the formation of paint dust.
Very dry air promotes the build-up of static electricity. Dust particles are then more easily attracted to the surfaces prepared for painting. With certain metallic paints, drying too quickly affects how the particles that give the coat its visual effect settle.
Air that is too humid
High humidity prolongs the evaporation of water from the paint and the time needed between coats. Depending on the material and the applied thickness, runs or differences in appearance sometimes occur.
The temperature of the parts must also be checked. Condensation forms on a part brought in from a cold hall if its surface is below the dew point of the booth air. The dew point is the temperature below which the water vapour in the air begins to turn into water on a cold surface.
The defects described also have other causes. Humidity is assessed together with paint viscosity, coat thickness, surface preparation and application parameters.
How do you check whether humidity affects the finish?
A paint defect alone does not indicate a humidity problem. It is worth investigating especially when quality variations occur at the same time as changes in the weather, the season or the temperature in the hall.
Check whether:
- defects are more frequent in winter, when outdoor air is heated before entering the booth, or on hot and humid days;
- the result differs between the morning shift and the afternoon shift, although the material and the settings have remained the same;
- the amount of paint dust in the booth or material consumption increases;
- operators adjust the application more often at certain times of the year;
- the number of parts that require sanding or repainting increases.
Record the temperature, humidity and defects observed per batch or per shift. Then compare the data with the material used, the spray gun parameters, air movement and part preparation. This way you can identify the factors that change during the periods when defects occur.
Why does the same spray gun setting give different results?
The spray gun setting influences the paint flow rate and the way the material is atomised. Booth conditions influence how much water the paint loses before and after it reaches the part.
Take a setting established during a period of high humidity. When the booth air becomes much drier, the paint loses water faster and reaches the part drier. The spray gun runs at the same parameters, but the conditions in which the coat forms have changed.
The documentation of some paint manufacturers indicates different adjustment directions:
Booth conditions | Effect on waterborne paint | What is analysed |
Warm and dry air | Water evaporates faster | An application that leaves enough time for the paint to flow out on the part |
Warm and humid air | Evaporation is slower | The amount of material applied per coat and air movement |
The table explains the principle, but settings are established according to the paint’s technical data sheet and the trials carried out on the line in question.
On automatic lines, the spraying parameters are set during commissioning. Walther Pilot automatic spray guns repeat the application under the same setting conditions. If the humidity in the booth varies, the spray gun continues to operate at the set parameters, but the behaviour of the paint changes. Maintaining the booth conditions helps preserve the results for which the process was set.
How is humidity controlled in the painting area?
Treating the air supplied to the booth
In installations equipped with an air handling unit, the humidifier can be integrated into the unit or into the ductwork. The air thus reaches the booth at the humidity set for the process.
Several technologies are analysed for this configuration:
- Steam humidification. Water is converted into steam and introduced into the airflow. Condair EL uses electricity, while Condair GS runs on gas. The choice takes into account the required capacity, the available energy source and the operating costs of the installation.
- Humidification & dehumidification
- Humidification & dehumidification
- Evaporative humidification. The air passes through a wetted material and takes up water by evaporation. Condair ME works on this principle. The unit does not spray droplets into the airflow and does not require an absorption distance after the evaporative module.
- Humidification & dehumidification
Direct room humidification
In sanding and preparation areas, humidity can be controlled directly in the work space. An adequate level reduces the build-up of static electricity and the attraction of dust particles to the parts.
Condair ML sprays water at high pressure directly into the air. The nozzles are positioned so that the droplets evaporate before reaching parts, equipment or other surfaces. The configuration depends on the size of the room and on air movement.
- Humidification & dehumidification
What does the choice of humidification system depend on?
To configure the installation, Texco specialists analyse several characteristics of the process:
- Booth air supply. The presence of an air handling unit and the ductwork configuration influence where the system is integrated.
- Materials applied. Waterborne and solvent-based paints have different requirements. The technical data sheets are checked for each stage, from primer to the final coat.
- Zone classification. If there are classified explosion-risk zones, the placement of the equipment is coordinated with the project documentation and the applicable requirements.
- Energy source. The availability of electricity or gas and the operating costs are part of the technical assessment.
- Seasonal variations. Outdoor air changes its temperature and water vapour content from one season to the next. The system is sized for the conditions it will encounter in operation.
- Water quality. The chosen technology determines the water treatment requirements and the auxiliary equipment needed.
How does Texco work?
Texco analyses the conditions in the painting area, the requirements of the materials used and the existing installation. Based on this data, Texco specialists configure the humidity control system, define its integration into the process and carry out the installation.
If the assessment also includes paint application, Texco proposes Walther Pilot spraying equipment suited to the material used and the requirements of the line.
Frequently Asked Questions
Why does relative humidity drop in the paint booth in winter?
Cold outdoor air generally contains less water vapour. When it is heated before entering the booth, relative humidity drops if humidity is not controlled. The humidification demand is therefore higher in winter in many installations.
What conditions are required in the drying phase?
Drying conditions sometimes differ from those required during application. Temperature, humidity, air movement and the time between coats are set according to the technical data sheet of the material and the process used.
How does humidification affect air temperature?
Evaporative humidification cools the air, because the evaporation of water consumes heat from the airflow. Steam humidification has a different effect on temperature, depending on the configuration of the installation. Both effects are taken into account when designing the air treatment for the booth.
What water is used for humidification?
The requirements depend on the chosen technology. Systems that spray water directly into the air require, depending on the application, demineralised water or water treated by reverse osmosis. This prevents minerals from being deposited on the parts prepared for painting.
Why is equipment checked for silicone compounds?
Traces of silicone compounds contaminate surfaces prepared for painting and can cause small craters in the applied coat. For nozzle humidifiers used in painting areas, the absence of silicone compounds from the relevant components must be included in the technical specification.
