Five Degrees Is the Floor, Not the Target

The Wet Edge, No. 002. Short field notes for coating crews.

Everybody on the crew knows the rule: steel has to be 5°F above the dew point. What gets missed is that 5°F is the floor. It's not a comfortable place to paint.

The rule shows up in ASTM D3276, ISO 8502-4 and plenty of specs, and it covers prep, application and cure. The margin is there for a reason. As solvent flashes off a fresh film, it pulls heat out of the surface, and cold material does the same thing. Steel that read 6°F over at 7 a.m. can be under by the second pot.

Why it matters: condensation you can't see is enough. Paint over it and you can get poor adhesion, blush on epoxies, or flash rust on fresh blast. None of it shows up until the inspector's knife or the first winter.

The practical move: read the steel, not the air, and read the coldest steel you're coating, like the shaded side or the north wall. Keep taking readings through the day. When the spread is shrinking toward 5°F, make the call before you're inside it. Some data sheets ask for more than 5°F, and if yours does, the data sheet wins.

Bottom line

Five degrees over the dew point is the minimum, not a safe margin. Read the coldest steel, watch the trend, and stop before the spread closes.

Further reading: Measuring Environmental Conditions for Coating Application (DeFelsko). Standards: ASTM D3276, ISO 8502-4.

Talk to a consultant. Questions on a job? Atlantic Coating Consultants supports contractors in PA, NJ, DE, MD, DC, WV and OH with product recommendations, field support and crew training. Call 484-644-5303 or email atlantic@tnemec.com.