It sounds obvious, but when painting outside always check what the weather forecast will be during the preparation, application and drying phases of your project. If fair weather is forecast, you should also take into account the air and surface temperatures, humidity and dew point to help you decide whether to start or not.
Relative humidity, or ‘RH’, measures the amount of water in the air in vapor form, comparing it to the maximum amount of water that can be held at a given temperature. For example, if the RH is quoted as being 50% at 73°F, this would imply that the air contains 50% of the maximum level of water vapor it could hold at 73°F. 100% RH indicates that the air is at maximum saturation.
When humid air comes into contact with cooler air, or a cooler surface, the water vapor will turn into water droplets. When this occurs on a surface it is referred to as the ‘dew point’.
Never apply paint above a maximum relative humidity of around 85%. At that level you’ll reach the dew point regardless of surrounding temperatures.
Relative humidity can be measured using a hygrometer. You could also use a hygro thermometer that measures both temperature and humidity.
This is the temperature at which the air can no longer hold its water vapor and starts to form water droplets i.e. condensation. Condensation forms when air temperature lowers, reducing the amount of energy available to keep the water in vapor format. For example, a window forms condensation when the cool night air hits the outside surface of the glass. This lowers its surface temperature, whilst the humidity in the house is high enough to cause condensation on the inside of the glass surface.
When painting, it’s essential that you’re able to paint a surface without condensation forming during the application and drying stages. The optimum surface temperature for paint application is normally around 6°F above the dew point. However, some products may be more or less tolerant. When working with International products, if in doubt, it’s always worth checking the relevant technical datasheet
The dew point can be calculated using the relative humidity and the air temperature. But to make life easier, you can find many dew point lookup tables on the web.
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