Inspecting & Accepting Industrial Coating Work — DFT per ISO 19840 / SSPC-PA 2, the 80-20 Rule, Adhesion and Holiday

See the whole-system guide: the Metal Fabrication / machine-shop fit-out guide — this article is one step — see the end-to-end fit-out guide.

However well a coating spec is written, it means nothing if you can't verify it on site — most industrial coating disputes aren't about "which paint" but about "is it thick enough, measured how, and accepted on what rule."

The primary metric is dry film thickness (DFT), because corrosion protection depends on achieving the full system thickness (see system selection at ISO 12944). This article covers how to measure it and the acceptance criteria to put in a TOR so handover doesn't end in argument.

1. DFT — the primary metric, measured correctly

DFT (Dry Film Thickness) = the coating thickness once fully cured, in microns (µm) — unlike WFT (wet), measured right after application.

Measured per ISO 2808 with a gauge:

• Magnetic / magnetic-induction — for coatings on steel (most common)
• Eddy-current — for coatings on aluminium/stainless

The gauge must be calibrated and a correction value set for surface roughness. A blast-cleaned profile makes the gauge over-read; you must subtract a correction value (typically 10–25 µm depending on profile) per ISO 19840, or you accept work that is thinner than it appears.

2. ISO 19840 acceptance — the "80-20 rule"

For an inspection area, all four must hold:

Criteria 2 and 3 together are the origin of the "80-20 rule" — readings may dip to 80%, but the number below NDFT must not exceed 20%.

3. SSPC-PA 2 — the gauge → spot → area structure

The American standard SSPC-PA 2 (AMPP) uses a layered counting method:

Criterion: a spot must be within 80%–120% of the specified value (per the chosen Coating Thickness Level).

The key difference: ISO 19840 emphasises statistical criteria (mean + 80% minimum + 20% rule), while SSPC-PA 2 emphasises the gauge/spot/area count + the 80–120% band. A TOR should cite one standard, not mix them.

4. Not just thickness — adhesion + holiday + surface condition

Full thickness ≠ good work; check three more dimensions:

flowchart TD
  A[Calibrate gauge + set profile correction] --> B[Measure DFT, many points per area]
  B --> C{Pass ISO 19840 / SSPC-PA 2?}
  C -->|No| X[Apply more / repair, re-measure]
  C -->|Yes| D[Adhesion test ISO 4624 pull-off]
  D --> E{Adhesion meets spec?}
  E -->|No| X
  E -->|Yes| F[Holiday detection for immersion/tank/pipe]
  F --> G[Surface check sag/run/dry-spray + ISO 4628]
  G --> H[Issue acceptance certificate]

• Adhesion — pull-off test per ISO 4624 (a dolly is pulled, failure force read in MPa) or cross-cut for thin films
• Holiday / pinhole — invisible pinholes found with a low-voltage (wet sponge) or high-voltage spark detector per NACE/ASTM, especially for immersion / tank / pipe work
• Surface condition — no sags, runs, dry spray or overspray, and rate degradation (rust/blister/crack/flake) per ISO 4628 on later inspections

5. What to put in a TOR so work can be accepted

• Cite the measurement standard: ISO 19840 or SSPC-PA 2 (pick one) + ISO 2808 for method
• State NDFT + maximum thickness per coat and total (consistent with the ISO 12944 system)
• State number of points/area and the calibration + correction-value procedure
• State adhesion test (ISO 4624) + minimum pass value (MPa)
• State holiday detection for immersion/tank/pipe + test voltage
• Require calibrated, certified instruments and who inspects (e.g. NACE/AMPP/FROSIO inspector)
• Require records: a DFT report per area + the ambient conditions (RH, dew point) during application

Tip: a good TOR states both "how thick" and "measured/accepted on what rule" — omit the latter and the inspector and contractor read it differently, ending in a dispute.

Summary

Good coating acceptance checks three dimensions: thickness (DFT) per ISO 19840 (the 80-20 rule) or SSPC-PA 2 (gauge/spot/area, 80–120%) + adhesion (ISO 4624) + holidays for immersion work — all after calibrating the gauge and subtracting the surface-roughness correction.

Stating the measurement standard and acceptance criteria clearly in the TOR from the start is the best way to avoid handover disputes — and to ensure the good ISO 12944 system you chose actually reaches its designed thickness.

Need corrosion-protection coating work that can be accepted to standard, with full DFT/adhesion reports — request a quote and our team works to ISO 19840 / SSPC-PA 2 and delivers the acceptance documentation.