If you want to paint outside, you should always consult the current weather forecast first. Not only during painting, but also during preparation and drying the weather must be good. And if good weather is forecast, factors such as air and surface temperature, relative humidity and dew point are important. You will often find useful information on the label of the paint can or in an enclosed product sheet. The ideal painting weather? 18 degrees and a cloudy sky.
Dew can adversely affect paint drying
Dew is critical in paint application. The evaporation of solvent requires heat and / or energy and cools the surface. Even though the conditions during paint processing are good, they can deteriorate during drying, which can lead to problems. Relative humidity is important because air can only absorb a certain amount of water or solvent vapor at a time.
If paint dries poorly, it is at the expense of the quality of the paint job
When the relative humidity increases, the air can absorb less solvent vapors and that slows down the drying of the paint. The temperature of the ambient air and the material to be treated therefore continuously influence the drying process. Failure to observe the necessary drying time can lead to disruptive defects in the quality of the paintwork.
Avoid extreme temperatures
- Low temperatures extend the drying time. So make sure that the paint has dried sufficiently before sanding or painting over again. Sanding too quickly can cause wrinkles in the paintwork, clogging up of the sandpaper and thus damage the paint layer. Overcoating too soon can result in blistering and loss of gloss;
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High temperatures shorten the drying time and adversely affect the paint job because the run-off properties are affected. In such cases, dilute the paint as described on the label or product sheet;
- Never apply the paint in direct sunlight or when the surface has warmed up. Both situations change the drying properties of the paint and lead to poor paint running, too fast drying and cracking in the paint layer and reduced gloss.
What is Relative Humidity ???
Relative humidity is referred to as the amount of water in vapor compared to the maximum amount of water that the air can absorb at the current temperature. Example: if a relative humidity of 50% is measured at 23 degrees Celsius, this means that the air at that moment has absorbed half the possible humidity at 23 degrees. 100% therefore means that the air is completely saturated.
Use a hygrometer
If moist air comes into contact with cooler air or cold surfaces, the water vapor condenses into water droplets. The air then reaches its dew point on paint surfaces. Never process paint at a relative humidity of more than 85%, because then you will quickly reach the dew point of the ambient temperature. Relative humidity can be measured with a hygrometer.
What is the dew point ???
We have already briefly been introduced to the concept of dew point. Condensation on the windows is a clear example of this. The dew point is the temperature at which air can no longer hold the water vapor and water droplets form. Condensation takes place when the temperature drops and therefore reduces the air’s ability to retain moisture. When painting, it is crucial to exclude condensation until the paint has completely dried. The optimum temperature of the substrate should be at least 6 degrees above the dew point, even if some paint products have more or less tolerance.
Other important points when processing lacquer and varnish
- Dry, well-ventilated conditions are ideal for paint and varnish application. Good ventilation supports the drying process and a dust-free environment will benefit the shine. However, avoid windy conditions;
- Avoid painting in the late afternoon or when humidity is above 80%, because paint reacts very negatively to moisture. Loss of gloss can also be caused by nighttime condensation or temperature changes;
- Dyeing or varnishing wood should not be done at rising temperatures of the substrate, as this can lead to blisters in the paint, which are caused by the breaking of the wood fibers. The best result is achieved at decreasing temperatures of the wood. That will absorb the paint better.