Half of all coating failures are a weather record nobody kept. Ours is written down, hour by hour, and handed over.
Coating failures are usually blamed on the product. Occasionally that is fair. Far more often the product performed exactly as specified and the conditions it was applied in were outside the window the specification assumed — and because nobody recorded the conditions, there is no way to demonstrate either the cause or the remedy.
A dew point log is the cheapest insurance policy in a refit. It costs a meter and a few minutes an hour.
The physics, briefly
Dew point is the temperature at which the air in the hall will begin to condense water. If the substrate is at or below that temperature, there is moisture on the surface, whether or not anyone can see it. Paint applied over that film may look perfect on the day and will lose adhesion later, from underneath, in a pattern that has nothing to do with where anyone touched it.
The rule every product data sheet states is a margin: the substrate must be held some degrees above dew point during application and through cure. Meeting the rule requires three numbers — air temperature, relative humidity and substrate temperature — and substrate temperature is the one most often assumed rather than measured. A steel hull that has been outside overnight lags the hall by hours.
The surface can be wet, in specification terms, on a day when the hall feels dry and everything looks clean.
What the log contains
- Air temperature and relative humidity, read in the hall, at the work, not at the door.
- Substrate temperature, measured on the vessel — the number that decides whether the margin is met.
- Calculated dew point and the margin above it at that hour.
- Product, batch number and mix ratio for whatever went on during that hour.
- Wet and dry film thickness, so the built system can be checked against what was specified rather than against what was intended.
Hourly, because conditions in a large hall are not constant across a working day, and because a single reading at the start of a shift tells you about the start of the shift.
Why the record leaves with the vessel
A coating log is only worth keeping if somebody outside the yard can read it. Handed over, it does three things. It lets the owner or their surveyor verify that the system was built as specified. It gives any future yard a starting point instead of a guess. And if something does fail, it makes the cause findable — which is the difference between a warranty conversation and an argument.
It also disciplines us. A number that is going to be read by someone else is recorded differently from a number that is going into a drawer. That is not a comfortable thing to admit, and it is true in every yard, including the good ones.
What it does not do
A log does not make a bad preparation good. Fairing, cleanliness, edge treatment and the adhesion of what is underneath still decide most of a coating's life. What the log does is remove one whole class of failure — the invisible, delayed, unattributable kind — from the list of things that can go wrong, and put a date and a number against every coat that was applied.
Written at the yard in Tuzla. Figures quoted are the ones we record on the job and hand over with the vessel.