When working with polyester and epoxy resins, one of the key factors affecting the quality of the coating is the dew point. Improper atmospheric conditions can cause surface defects, such as clouding, weakened adhesion, or even coating failure. In this article, we will explain what the dew point is, how to calculate it, and how to prevent problems when working with resin.
1. What is the dew point?
The dew point is the temperature at which water vapor condenses on a surface, forming a layer of moisture. If the surface temperature falls below this value, water will begin to condense on its surface. This is crucial when applying resins, because even a small amount of moisture can disrupt the curing process and impair the adhesion of the resin layer.
The dew point depends on two factors:
- Air temperature – the higher the temperature, the more water vapor the air can hold.
- Relative humidity – the higher the humidity, the faster condensation occurs.
2. Why is the dew point important when working with resin?
Working in improper atmospheric conditions, especially when the surface is cooler than the dew point, leads to problems such as:
- Resin clouding – the coating does not cure evenly, and its surface becomes milky.
- Weakened adhesion – the resin does not bond well to the substrate, which can lead to it peeling off.
- Air bubbles – moisture trapped in the resin can cause defects.
- Curing problems – excess moisture can disrupt the polymerization process, preventing the resin from achieving full strength.
3. How to calculate the dew point?
The dew point can be calculated based on the air temperature and relative humidity using special calculators or tables.
Example dew point table:
| Air temperature (°C) | Humidity 40% | Humidity 50% | Humidity 60% | Humidity 70% | Humidity 80% | Humidity 90% |
|---|---|---|---|---|---|---|
| 10°C | -2.0°C | 1.0°C | 3.6°C | 5.9°C | 7.8°C | 9.3°C |
| 15°C | 3.1°C | 6.4°C | 9.0°C | 11.3°C | 13.2°C | 14.7°C |
| 20°C | 7.3°C | 10.8°C | 13.6°C | 15.9°C | 17.8°C | 19.3°C |
| 25°C | 11.5°C | 15.2°C | 18.1°C | 20.6°C | 22.5°C | 24.0°C |
Example:
If the air temperature is 20°C and the relative humidity is 70%, then the dew point is 15.9°C.
This means that if the surface on which we apply the resin is below 15.9°C, water may condense on it! In such a case, application should be avoided.
4. How to monitor the dew point when working with resin?
Measuring temperature and humidity
-
- Use a hygrometer to check the air humidity.
- Check the surface temperature using a pyrometer (a non-contact infrared thermometer).
Ensure an adequate temperature difference
-
- The substrate temperature should be at least 3°C higher than the dew point.
- Otherwise, moisture may condense on the surface, leading to adhesion problems.
Avoid application in improper conditions
-
- Do not apply resin at temperatures below 15°C and at humidity levels above 80%.
- If humidity is high, increase ventilation or use dehumidifiers.
Preheat the surface
-
- If the surface temperature is below the dew point, it can be gently preheated (e.g. with an air heater or warm air from a construction dryer).
Avoid drafts and sudden temperature changes
-
- Temperature fluctuations can lead to moisture condensation.
5. Summary – key rules for working with resin and the dew point
✔ Always check the temperature and humidity before applying resin.
✔ Maintain the surface temperature at least 3°C above the dew point.
✔ Do not apply resin in overly humid conditions (above 80% humidity).
✔ If necessary, preheat the surface or improve ventilation.
✔ Use a hygrometer and dew point table to avoid curing and adhesion problems.
💡 By consciously monitoring the dew point, you can avoid surface defects in the resin and ensure its excellent durability and appearance!
