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What is the 3°C dew point rule for painting?

Per NACE/SSPC and most coating specifications, the steel surface temperature must be at least 3°C (5°F) above the dew point before blasting or painting can begin. This prevents moisture condensation on the surface which would cause coating failure.

How is dew point calculated?

Dew point is calculated using the Magnus formula: γ = ln(RH/100) + (17.62×T)/(243.12+T), then Td = 243.12×γ / (17.62-γ). This gives the temperature at which air becomes saturated and water begins to condense.

What happens if you paint below the dew point?

If the steel surface is at or below the dew point, moisture condenses on the surface. This trapped moisture causes osmotic blistering, poor adhesion, and premature coating failure. Most specifications mandate work stoppage until conditions improve.

Dew Point Calculator for Coating Surface Readiness

Ambient Temperature (°C)

Relative Humidity (%)

Steel Surface Temp (°C)

The 3°C Rule

NACE/SSPC and most client specifications (Aramco SAES-H-001, ADNOC AGES-C-04) require the steel surface temperature to be at least 3°C above the dew point before abrasive blasting or paint application. If the difference is less than 3°C, work must stop until conditions improve.

Measurement Best Practices

Use a whirling hygrometer (sling psychrometer) or digital psychrometer for accurate RH readings. Measure steel surface temperature with a contact thermometer. Check conditions every 2-4 hours and whenever weather changes. Document all readings in the daily coating log.

The Magnus Formula Explained

Dew point is the temperature at which water vapor in the air begins to condense into liquid. The Magnus-Tetens approximation calculates it from ambient temperature and relative humidity. The formula is: γ = ln(RH/100) + (17.62 × T) / (243.12 + T), then Td = 243.12 × γ / (17.62 − γ). This calculator implements this formula directly and gives you the dew point to one decimal place.

How to Use This Tool on Site

Measure the ambient air temperature with a sling psychrometer or digital psychrometer.

Measure the relative humidity (RH) at the same location.

Measure the steel surface temperature with a contact thermometer.

Enter all three values — the calculator shows the dew point and whether it is safe to blast and paint.

Re-check conditions every 2–4 hours and whenever weather changes. Document all readings.

Why the 3°C Rule Exists

The 3°C buffer accounts for measurement uncertainty, micro-climate variations near the surface, and the fact that steel temperature can drop rapidly at night or when weather shifts. Even if the surface is 1°C above the dew point now, a slight temperature drop could cause condensation. The 3°C margin provides a safety buffer that virtually all coating specifications — NACE, SSPC, Aramco SAES-H-001, ADNOC AGES-C-04 — mandate.

What to Do When Conditions Fail

If the surface is less than 3°C above the dew point, work must stop. Options include waiting for conditions to improve (often midday in humid climates), using industrial dehumidifiers and heaters to raise the surface temperature, or enclosing the work area. Never proceed with blasting or painting when the 3°C rule is not met — the coating manufacturer will not warrant a system applied below this threshold.