The temperature at which water vapor in the air condenses to liquid. If steel is at or near the dew point, an invisible film of moisture forms on it, causing flash rust on blasted steel and poor adhesion under coatings. Specs nearly always require the surface to be at least 3 C (5 F) above dew point, often with relative humidity below 85 percent, and inspectors check it with a psychrometer or digital gauge before and during the work.
Steel's at 11 C and dew point is 9, that's under the 3 degree spread, nobody sprays till it warms up.
Take the steel temperature on the shaded side and the underside of the structure, not the sunny face, because that is where condensation forms first.
Also heard as
- dew point spread
- 5 degree rule
Related terms
Flash rust
Thin, quick layer of rust that forms on freshly blasted or water-washed steel within minutes to hours.
Amine blush
Waxy, greasy film on curing amine epoxies in cold or damp conditions that blocks adhesion of the next coat.
Near-white blast
Blast grade allowing light shadows or stains on no more than 5 percent of the surface, the usual immersion standard.
Pot life
Working time after mixing a two-part coating before it has reacted too far to apply properly.