What a coating inspection report has to record, and why dew point decides the day
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Short answer
A coating inspection report has to record four things to be defensible: the environmental conditions at the time of application (ambient temperature, surface temperature, relative humidity and dew point), the surface preparation grade achieved, dry film thickness readings against the specified minimum, and any holiday or pinhole testing. Dew point is the one that most often invalidates the work, because applying coating to a surface at or near the dew point traps moisture under the film. The common specification rule is that surface temperature must sit at least 3 degrees C above dew point.
Key numbers
- Environmental record
- Ambient temperature, surface temperature, relative humidity, dew point, wind and weather
- Dew point rule (typical specification)
- Surface temperature at least 3 degrees C above dew point
- Preparation grades
- Sa 1, Sa 2, Sa 2.5 and Sa 3 for abrasive blasting; St 2 and St 3 for hand and power tool cleaning (ISO 8501-1)
- DFT record
- Location, coat, reading in microns, and the specified minimum for that coat
- Holiday test
- Pass, fail or not tested, recorded per area
Environmental conditions are the record that cannot be reconstructed
Almost everything else on a coating report can be verified after the fact. Dry film thickness can be re-measured, preparation grade can be photographed, adhesion can be re-tested. Environmental conditions cannot: once the coating is on, there is no way to prove what the relative humidity was when it went on.
That is why environmental readings are the first thing an auditor looks for and the most common reason a coating record is rejected. They need to be taken at the work face, not from a weather station, and recorded against a time rather than a day.
- Ambient temperature
- Air temperature at the work location
- Surface temperature
- Substrate temperature, which can run well above air temperature in direct sun
- Relative humidity
- Percentage, measured at the work face
- Dew point
- Derived from ambient temperature and relative humidity
- Wind and weather
- Relevant to overspray, dry spray and contamination
How dew point is calculated
Dew point is the temperature at which air becomes saturated and moisture condenses. It is not measured directly in the field: it is derived from ambient temperature and relative humidity, usually with the Magnus approximation.
ForgeQM computes it in the inspection form as you enter temperature and humidity, so the delta against surface temperature is visible while the inspector is still standing at the job rather than after the coating has been applied.
- Step 1
- alpha = (17.27 x T) / (237.7 + T) + ln(RH / 100)
- Step 2
- dew point = (237.7 x alpha) / (17.27 - alpha)
- Where T is
- Ambient temperature in degrees C
- Where RH is
- Relative humidity as a percentage
- The check that matters
- Surface temperature minus dew point, against the margin in the specification
Surface preparation grade is a visual comparison, not a measurement
Preparation grades under ISO 8501-1 are defined by photographic comparators rather than by a numeric threshold. Sa 2.5, the most commonly specified grade for industrial coatings, is described as very thorough blast cleaning where the surface is free of visible contamination and any remaining traces appear only as slight stains.
Because it is a visual judgement, it should be photographed and recorded against a specific area. SSPC and AMPP designations are frequently specified alongside or instead of ISO grades, and they are close equivalents rather than identical.
- Sa 1: light blast cleaning
- Sa 2: thorough blast cleaning
- Sa 2.5: very thorough blast cleaning, the usual industrial specification
- Sa 3: blast cleaning to visually clean steel
- St 2 and St 3: thorough and very thorough hand and power tool cleaning
- SSPC-SP 1 solvent cleaning, SSPC-SP 6 commercial blast, SSPC-SP 10 near white blast
Dry film thickness has to be recorded against a specified minimum
A dry film thickness reading on its own means nothing. It only becomes a conformance record when it is recorded against the minimum specified for that coat at that location, so a reading of 180 microns is a pass on a 150 micron primer and a failure on a 200 micron intermediate.
ForgeQM records dry film thickness as repeatable rows carrying location, coat, the reading in microns and the specified minimum, and evaluates each reading against its own minimum. Statistical conformity assessment across a batch, of the kind ISO 19840 and AS 3894.3 describe, is not automated: readings are captured and compared individually, and the batch level judgement stays with the coating inspector.
What ForgeQM records on a coating inspection report
Coating inspection reports are auto numbered in a CIR series and move from draft to submitted through a validated transition, so a submitted report cannot be quietly edited afterwards.
- Environmental block with automatic dew point calculation
- Surface preparation grade from an ISO 8501-1 and SSPC picker
- Repeatable dry film thickness rows with per row minimum and pass or fail
- Holiday and pinhole test result as pass, fail or not tested
- Photographs attached through the inspection photo pipeline
- Draft to submitted status workflow with validated transitions
Frequently asked questions
Why does surface temperature have to be above dew point?
If the substrate sits at or below dew point, moisture condenses on it. Coating applied over that condensation loses adhesion and traps water against the steel, which causes blistering and under film corrosion that may not appear for months. The commonly specified margin is 3 degrees C, which allows for the surface cooling slightly during application and for instrument tolerance. Always use the margin in the project specification rather than assuming 3 degrees C.
Is Sa 2.5 the same as SSPC-SP 10?
They are close equivalents and are often cross referenced, but they are defined by different bodies against different comparators and are not formally identical. If the specification names one, record that one. Recording a different designation because it is thought equivalent is a common audit finding.
How many dry film thickness readings are enough?
The number and distribution come from the project specification and the standard it references, commonly ISO 19840 or AS 3894.3, which define measurement areas and how readings are assessed against a minimum. There is no universal number. What matters for the record is that each reading is tied to a location and a coat, so the sampling can be shown to be representative rather than selective.
Does ForgeQM decide whether a coating system passes?
Not at the batch level. Each dry film thickness reading is evaluated against the minimum entered for that coat, and the environmental readings are computed and displayed. A coating requirements checker (beta) is available for entitled Coating and Checkers packs: dew point margin, surface preparation, holiday testing and inspection frequency guidance under ISO 12944 and AS/NZS 2312.1. It does not hold manufacturer DFT tables or compute statistical conformity under ISO 19840 or AS 3894.3. The professional judgement stays with the coating inspector.
Sources
This guide is written against the published standards below. Standards are revised, and project specifications override general guidance, so confirm clause numbers against the edition your contract names before relying on them.
- ISO 8501-1 Preparation of steel substrates before application of paints and related productsVisual assessment of surface cleanliness, and the Sa and St preparation grade definitions.
- ISO 12944 parts 1 to 8, Paints and varnishes, Corrosion protection of steel structures by protective paint systemsCorrosivity categories, surface preparation, paint system selection and execution of work.
- AS 3894 series, Site testing of protective coatingsSite test methods including dry film thickness measurement (AS 3894.3) and holiday detection.
- AS/NZS 2312.1 and AS/NZS 2312.2Guide to the protection of structural steel against atmospheric corrosion by protective coatings.
Related guides
- MDR and ITP: Building a Fabrication Handover PackAn ITP plans the inspection, an MDR proves it happened. What a manufacturing data record contains, how it maps back to the ITP hold and witness points, and why compiling it at the end of a job is what makes handover painful.
- Weld NDT Requirements and Inspection ExtentsVisual scanning is always 100%. Beyond that, NDT extent comes from the governing standard, the weld category or construction class, and the material. AS/NZS 1554 structural rules and a compact AS 4037 class table, with method selection logic.
- WPS vs PQR vs WQR: What Each Record ProvesA PQR records a test weld and its results, a WPS turns that evidence into welding instructions with qualified ranges, and a WQR proves an individual welder can work to it. Written order, what carries between them, and the AS/NZS 1554.1 ranges.
Stop checking this by hand
ForgeQM applies the AS/NZS 1554 series as checks while you write the record, not after. Qualification ranges are generated from the PQR, NDT extents come back with the clause they came from, and job data carries across every document through to the MDR.
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