In the painting and corrosion protection of prefabricated steel structures (PEB / Heavy Steel), the dew point temperature is one of the most critical environmental indicators in determining whether spray painting operations can be carried out.
I. Environmental Parameters and Control Data
When painting prefabricated steel structures in a spray booth or on-site, the following four key environmental parameters must be monitored simultaneously:
| Monitored Parameter | Standard Control Range | Notes & Execution Guidance |
| Steel Surface Temperature vs. Dew Point Difference (Ts – Td) | ≥3°C | Mandatory Red Line. Spraying must cease immediately if the delta drops below ≥3°C. |
| Relative Humidity (RH) | ＜85% | Typical epoxies and polyurethanes require RH< 85%. Inorganic zinc-rich primers may allow 80%-90% (moisture aids curing). |
| Ambient Air Temperature (Ta) | 5°C～35°C | Below 5°C, epoxy curing halts; above 35°C, rapid solvent evaporation leads to dry spraying and defects. |
| Steel Surface Temperature (Ts) | 5°C～40°C | Must be measured directly on the steel substrate using surface contact thermometers, not estimated from air temp. |
II. Practical Measurement and Calculation Methods
Factory quality control personnel (QC / NACE inspectors) typically follow these steps to take measurements before starting work:
2.1 Measure ambient temperature and humidity: Use a dry-and-wet-bulb hygrometer or a digital temperature and humidity meter to measure the current ambient temperature (Ta) and relative humidity (RH) in the workshop.
2.2 Determine or Calculate the Dew Point (Td): Read the current dew point temperature directly from a “Dew Point Reference Table” or a digital dew point meter.
Example: If the workshop temperature is 25°C and the relative humidity is 70%, consulting the table reveals that the dew point temperature is approximately 19.1°C.
2.3 Measure the Steel Plate Surface Temperature (Ts): Use an infrared thermometer or a contact-type magnetic surface thermometer to measure the surface temperature of the steel component.
2.4 Determine Compliance:
Based on the example above, the steel plate surface temperature must reach at least 19.1°C + 3°C = 22.1°C before spraying can begin.
III. Quality Control Recommendations for Coating Prefabricated Steel Structures
3.1 Morning-Evening Temperature Differences and Early Morning Construction
Before work begins in the morning, steel components have cooled overnight, and their surface temperatures are often lower than the ambient temperature. Even when the morning air is dry, the surface of steel plates can easily drop below the dew point (causing surface condensation); therefore, special attention must be paid to temperature measurements in the early morning.
3.2 Time Interval Between Sandblasting and Coating (Window Time):
After sandblasting for rust removal (e.g., Sa2.5) is completed, if the dew point difference is less than 3°C, the exposed steel will develop “flash rust” within a few minutes. Sandblasting and primer application are strictly prohibited when humidity is high or the dew point does not meet specifications.
3.3 Archiving of Quality Inspection Records:
When maintaining records for the factory quality management system, the “Environmental and Dew Point Measurement Record Form (Environmental Inspection Log)” must be completed before the start of each workday to record the current air temperature, relative humidity, dew point, and steel plate surface temperature. This constitutes an extremely important anti-corrosion QA/QC record.