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Specialist suppliers to
the Blast Cleaning &
Coating Industry

Blast-One Weekly Tips
Tip #55
Are the atmospheric conditions okay to paint today?

It is critical to the success of most coating systems that the surface is completely free of moisture prior to and during paint application and curing.

Condensation of water (dew) from the atmosphere on to the surface will occur, given the right conditions.

For any given set of conditions, the temperature at which condensation will occur is called the Dewpoint. As long as the surface temperature is 3°C/5°F (or more) above the Dewpoint temperature, it is generally considered safe to paint as far as risk of condensation is concerned.

How to use the chart to calculate dewpoint

1. Measure
• Air temperature
• Surface temperature
• Relative humidity
Use the same instrument for reading the air and surface temperature, and with an accuracy of ±0.5°C.

2. Calculate the temperature difference i.e. air temperature minus surface temperature.

3. Plot and intersect on the chart the temperature difference and the relative humidity.
If the intersection point is BELOW the guide line
• Indicates conditions are safe to paint

If the intersection point is ABOVE the guide line
• Indicates conditions are UNSAFE for painting

If you would like to calculate this automatically use a Whirling Hygrometer or a PosiTector Dew Point Meter.


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