Painting and the Dew Point: the 3 °C Rule (ISO 8502-4)

Thin films of condensed water on a surface are mostly invisible, which is why ISO 8502-4 exists: it gives guidance on estimating the probability of condensation before painting^[1][2].

The rule

Unless otherwise agreed, the steel surface temperature should generally be at least 3 °C above the dew point when paints are used. Moisture-tolerant paints may allow less, and the paint manufacturer or project spec may set other values^[2].

Procedure (summary of ISO 8502-4)

1. Measure air temperature and relative humidity near the surface.
2. Calculate the dew point (use the calculator).
3. Measure the surface temperature with a contact thermometer.
4. Check the margin — and consider conditions over the application and drying period, which the standard notes is usually about 6 hours^[2].

Instrument accuracy named in the standard

• Air thermometers accurate to ±0.5 °C^[2].
• Hygrometers accurate to ±3% RH (e.g. capacitive polymer sensors)^[2].

Scope note: ISO 8502-4 is written for steel substrates; many coating manufacturers apply the same 3 °C margin on other substrates, but always follow the product data sheet.

Sources

1. ISO 8502-4:2017 – estimation of the probability of condensation prior to paint application
2. ISO 8502-4:2017 public preview (iTeh)
3. Alduchov & Eskridge (1996), Improved Magnus form approximation of saturation vapor pressure, J. Appl. Meteor.