Dew point: when to stop painting
Applying coating to steel that is at or near the dew point is one of the most common causes of early coating failure. Moisture you cannot see condenses on the surface, flash rusts the steel or interferes with adhesion, and the failure shows up months later as blistering or delamination.
The rule most specifications apply: surface temperature at least 3 degrees Celsius (5 degrees Fahrenheit) above the dew point, and relative humidity at or below 85 percent, during application and initial cure. Your specification governs; some products carry different limits.
What the dew point is
The dew point is the temperature at which the moisture in the air condenses into liquid. It depends on air temperature and relative humidity: warm humid air has a high dew point, cold dry air a low one.
CoatSpec calculates it with the Magnus formula using the Alduchov-Eskridge constants (a = 17.625, b = 243.04), a fit accurate to a small fraction of a degree across the range coating work happens in. At 25 C and 60 percent RH the dew point is about 16.7 C.
Why the spread rule exists
Condensation does not wait for the surface to reach the dew point exactly. Surface temperature readings carry error, conditions drift, and a molecular moisture film forms before visible dew. The 3 C spread is a margin against all three.
The dangerous windows are predictable: early morning when steel is still cold from the night, evenings when air cools faster than steel warms, sun-to-shade transitions, and any time humidity climbs toward saturation. Steel in the shade can sit several degrees below the air temperature.
Logging conditions defensibly
Measure at the work location, not at the trailer. Record air temperature, surface temperature, relative humidity, the calculated dew point, the spread, the instrument used, and the time. Repeat at least every four hours and whenever conditions visibly change.
A hold call is easier to defend with a log. CoatSpec timestamps each snapshot, computes the spread against your configured criterion, and keeps the log with the work area it belongs to.
Frequently asked questions
What is the 3 degree rule in painting?
Common field practice requires the surface temperature to be at least 3 degrees Celsius (5 degrees Fahrenheit) above the calculated dew point before and during coating application. It is a safety margin against condensation forming on the surface as conditions drift.
What is the dew point at 25 C and 60 percent humidity?
About 16.7 C by the Magnus formula. A steel surface at 19.7 C or warmer would satisfy the common 3 C spread requirement at those conditions.
Can I paint when relative humidity is above 85 percent?
Most specifications hold work above 85 percent RH even when the dew point spread is adequate, and some coatings (moisture-cured products are the notable exception) have their own tighter limits. Check the specification and the product data sheet.