# Dew point and the 3 °C margin Source: https://coatmentor.com/learn/dew-point/ · Updated 2026-10-04 What the dew point is, why steel must be at least 3 °C above it before blasting and coating, and how inspectors measure it. *Figure: Example. Coat only when the steel is at least 3 °C above the dew point.* The dew point is the temperature at which water vapour in the air condenses. A steel surface colder than the dew point gets wet, often with a film too thin to see. For the climate rules of each standard side by side, read the [dew point and RH guide](https://coatmentor.com/blog/dew-point-relative-humidity-steel-temperature). ## The physics of condensation Air holds a limited amount of water vapour, and warm air holds more than cold air. Relative humidity (RH) says how full the air is at its own temperature. At 100 % RH the air is saturated. Cool a sample of air and its vapour content stays the same, but its RH rises. At the dew point the RH reaches 100 %. Below that temperature the air cannot keep all its vapour, and liquid forms on the first cold surface it touches. The dew point depends on the water in the air. It does not depend on the surface. If you heat a closed space and add no water, the air temperature rises, the RH falls and the dew point stays the same. Steel and air rarely share one temperature. Steel has mass, so it changes temperature slowly. At night steel radiates heat and cools below the air temperature. In the day the sun warms it above the air temperature. A thick section lags behind the air in both directions. For this reason the inspector measures the steel and not only the air. ## Why 3 °C and why 85 % RH ISO 8502-4 asks for steel at least 3 °C above the dew point, unless the parties agree otherwise. SSPC-PA 1 sets 5 °F instead, which is a little less: 3 °C is 5.4 °F. A paint made for damp surfaces can allow less. A data sheet can ask for more. The margin gives room for instrument errors and for steel that is colder in one place than in another. The same standard shows how little cooling humid air needs before it condenses. At 90 % RH a drop of 1.6 °C is enough. At 98 % RH a drop of 0.3 °C is enough. At 85 % RH or above, the standard says to judge painting critically. Many specifications therefore set two limits together: RH not above 85 % and steel at least 3 °C above the dew point. NORSOK M-501 (Edition 7) and the IMO PSPC for ballast tanks both use this pair. Wet steel harms the coating in three ways. A blasted surface flash rusts. A fresh coat on a damp surface bonds badly. Condensation on a coat that has not cured can cause blush and poor cure, as [amine blush](https://coatmentor.com/defects/amine-blush/) shows. Soluble salts can hold moisture at a lower RH than clean steel does, so test for them as well ([soluble salts](https://coatmentor.com/learn/soluble-salts/)). ## In the field 1. Measure the air temperature and the RH close to the work. Use a psychrometer ([ASTM E337](https://coatmentor.com/standards/astm-e337/)) or an electronic meter. The aspirated psychrometer is more accurate than the sling type. 2. Find the dew point from the tables or from the ISO 8502-4 formula. The [dew point calculator](https://coatmentor.com/calculators/dew-point/) uses the same formula. 3. Measure the steel temperature with a contact thermometer on the surface you will coat. Take at least one reading for each 10 m² and use the lowest. Include the coldest places, such as shaded and thick parts. 4. Do not use an infrared thermometer. ISO 8502-4 advises against it. 5. Subtract the dew point from the lowest steel temperature. If the result is below the limit, stop work and record the time. 6. Repeat the readings at the intervals the specification sets and whenever the weather changes. Keep the limits through application and the early cure. Instruments need a known accuracy. ISO 8502-4 gives ±0.5 °C for thermometers and ±3 % RH for psychrometers and capacitive hygrometers. ## A worked example The air is 22 °C at 60 % RH. The dew point is 13.9 °C. The specification asks for a 3 °C margin, so the steel must be at least 16.9 °C. Four steel readings are 19.0, 17.8, 16.4 and 18.2 °C. The lowest reading governs. The margin is 16.4 − 13.9 = 2.5 °C, so the work waits. Check: 13.9 + 3 = 16.9 °C, and 16.4 °C is below that limit. Later the air stays at 22 °C but the RH rises to 70 %. The dew point rises to 16.3 °C, so the steel must now be at least 19.3 °C. A rise of 10 % RH moved the limit by 2.4 °C. ## What the inspector records - The date, the time and the place of each set of readings. - The air temperature, the RH and the dew point. - Every steel reading and the lowest one. - The instruments used, with their calibration status. - The limit from the specification or data sheet, and the decision: work released or stopped. ## Exam traps - Trap: using the air temperature for the margin. The margin is the steel temperature minus the dew point. - Trap: averaging the steel readings. Use the lowest one. - Trap: measuring the steel with an infrared gun. - Trap: believing that heating the air lowers the dew point. It lowers the RH only, unless moisture leaves the air. - Trap: trusting a surface that looks dry. A film of condensation can be too thin to see. - Trap: checking only at the start. The limits also hold during application and the early cure. ## How the exams treat it The AMPP CIP Level 1 guide names ASTM E337. The CIP 1 practical has an environmental test station worth 20 of 100 points. The CIP 2 practical has an environmental testing station worth 10 points. The FROSIO book covers ambient conditions. For the full comparison of limits and borderline conditions, read the guide [Dew point, RH and steel temperature](https://coatmentor.com/blog/dew-point-relative-humidity-steel-temperature). **Use it:** [Dew point calculator](https://coatmentor.com/calculators/dew-point/) · [Dew point, RH and steel temperature](https://coatmentor.com/blog/dew-point-relative-humidity-steel-temperature) **91** practice items in the members area mention this topic (CIP 1: 39 · CIP 2: 28 · FROSIO written: 24). ## Related - **Standards:** [ISO 8502](https://coatmentor.com/standards/iso-8502/) · [ASTM E337](https://coatmentor.com/standards/astm-e337/) · [SSPC-PA 1](https://coatmentor.com/standards/sspc-pa-1/) · [IMO PSPC](https://coatmentor.com/standards/imo-pspc/) - **Defects:** [Blistering](https://coatmentor.com/defects/blistering/) · [Flaking and delamination](https://coatmentor.com/defects/flaking/) · [Amine blush](https://coatmentor.com/defects/amine-blush/) · [Blushing (solvent blush)](https://coatmentor.com/defects/blushing/) - **Conditions and application:** [Volume solids, wet film and dry film](https://coatmentor.com/learn/volume-solids/) · [Pot life and induction time](https://coatmentor.com/learn/pot-life/) · [Drying, curing and recoat windows](https://coatmentor.com/learn/cure-and-recoat/) · [Stripe coats](https://coatmentor.com/learn/stripe-coat/) ## About this page **Sources:** ISO 8502-4:2017 (probability of condensation before painting) · ASTM E337-15(2023) (humidity by psychrometer) · NORSOK M-501:2022, Edition 7, cl. 5.4 · IMO MSC.215(82) (PSPC for ballast tanks) · DeFelsko, Measuring environmental conditions for the application of paints and coatings. Written from the published standards and coating practice, in our own words, and checked on 2026-10-04. Found an error? It goes in the [corrections log](https://coatmentor.com/method/). CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.