Dew point, relative humidity and steel temperature: the climate rules for coating work
Condensation you cannot see is one of the commonest reasons a coating fails early. That is why every coating specification sets climate limits, and why both the AMPP CIP and FROSIO exams test them. This guide covers the rules, how to measure the conditions correctly, and includes a calculator that uses the same formula as ISO 8502-4.
The short answer: unless the specification or data sheet says otherwise, the steel surface must be at least 3 °C above the dew point (ISO 8502-4; SSPC practice uses the same 5 °F / 3 °C margin). Treat 85 % relative humidity or more as a warning: at 85 % the dew point is only about 2.5 °C below air temperature. The limits have to hold through application and drying, not only when you take the reading.
Dew point calculator
Uses the Magnus-type formula from ISO 8502-4 Annex A (valid for air temperatures of 0 °C and above at normal pressure). A planning aid, not a substitute for a calibrated instrument or your specification's limits.
The four readings and what they mean
- Air temperature — the dry-bulb temperature of the air near the work.
- Relative humidity (RH) — how much water vapour the air holds, as a percentage of the most it could hold at that temperature.
- Dew point — the temperature at which that air becomes saturated. Any surface at or below it will collect condensation.
- Steel surface temperature — the reading that decides the question. What matters is the gap between the steel temperature and the dew point, not the air temperature.
Why the margin is 3 °C
Steel temperature is never uniform, instruments have tolerances, and conditions change while the paint dries. ISO 8502-4 therefore sets a 3 °C margin unless something else is agreed, and allows a smaller one only for paints designed to tolerate surface moisture, where the manufacturer says so. The standard also shows how small the temperature drop to condensation becomes as humidity rises:
| Relative humidity | Temperature drop before condensation (approx.) |
|---|---|
| 80 % | 3.4 °C |
| 85 % | 2.5 °C |
| 90 % | 1.6 °C |
| 92 % | 1.3 °C |
| 95 % | 0.8 °C |
| 98 % | 0.3 °C |
At 92 % RH the standard advises painting only if conditions will clearly hold steady or improve through application and drying, a period it puts at about six hours. Even at a comfortable 80 %, you still check the forecast for that window.
How to measure it properly (ISO 8502-4)
- Air temperature and humidity: use a thermometer accurate to ±0.5 °C, and either a sling or aspirated psychrometer with its tables or an electronic hygrometer. ASTM E337 is the equivalent psychrometer method in US practice. Electronic sensors drift and are easily contaminated, so keep their calibration current.
- Steel temperature: use a contact surface thermometer accurate to ±0.5 °C and leave it on long enough to settle. Take at least one reading per 10 m², and use the lowest reading in the dew point check. ISO 8502-4 advises against infrared thermometers for this measurement.
- Where to measure: shaded faces, thick sections and areas near cold ballast or seawater lag behind the air temperature. Measure where the steel is coldest, not where it is easiest to reach.
- Record it: readings, times, locations and instruments go in the daily report. Where a data logger runs overnight, check it against your calibrated instrument.
Borderline conditions to watch
- Evening and clear nights: steel radiates heat to a clear sky and can drop below the air temperature, and below the dew point, after sunset.
- Heavy sections: a thick plate warms slowly in the morning, so it can stay below the dew point while thinner steel nearby is fine.
- Salty surfaces: hygroscopic salts pull moisture out of the air below 100 % RH, which is one more reason to test for soluble salts before painting.
- Product-specific limits: the data sheet can be stricter than the 3 °C rule, and moisture-curing products such as ethyl-silicate zinc also need a minimum RH to cure.
How the exams test it
Expect questions that give an air temperature and RH and ask whether work can proceed at a stated steel temperature. You may also be asked which of several steel readings governs (the lowest one), why an infrared gun is the wrong tool here, or what happens to the dew point when you heat an enclosure without removing moisture. It stays the same, while RH falls. Practise with the calculator until you can estimate the answer before you compute it.
Climate rules come up in both CoatMentor tracks: the CIP multiple-choice banks and the FROSIO written prompts, where you have to write out the reasoning. Try the free sampler and see how you do on them.
Try the free sampler See pricing →Sources
- ISO 8502-4:2017 — Guidance on the estimation of the probability of condensation prior to paint application — the 3 °C margin, RH guidance, instruments, measurement procedure and the Annex A dew point formula
- ISO 8502-4:1993 (previous edition) — for readers whose specification still cites it
- DeFelsko — Measuring environmental conditions for the application of paints and coatings — the 5 °F / 3 °C margin in ISO and SSPC practice
Checked on 23 September 2026.
Related guides: Surface preparation cleanliness grades · DFT measurement: SSPC-PA 2 vs ISO 19840 · What is the FROSIO exam?.
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