Why the margin exists
Condensation does not need to be visible to do damage. A molecular film of moisture on freshly blasted steel starts flash rusting immediately and interposes itself between substrate and primer; the coating applied over it is bonded to a weak boundary layer, not to the steel. The margin above dew point exists because surface temperature and ambient conditions both move — a cold morning, a shaded face, a wind shift — and the margin absorbs that movement.
How it is checked
The site kit is simple: surface thermometer (or IR gun used correctly), whirling or electronic hygrometer for ambient temperature and relative humidity, and psychrometric tables or the instrument's calculation for the dew point. Readings are taken where the work is — the coldest substrate governs, not the datalogger at the office — and repeated through the shift, because conditions at 7 a.m. and 2 p.m. are different climates.
The scheduling consequence
In Quebec and Ontario, dew point discipline is a seasonal planning constraint: spring and fall shifts often lose their early hours to condensation conditions, and heated enclosures or dehumidification become part of the quote, not an extra. A coating failure investigated months later very often traces back to a shift where nobody wrote down these three numbers.