The bottom line
Facing the same question — "how do you judge a thick coating on a blasted steel structure acceptable" — the US and Europe each have a rule set. The two judge differently — ISO 19840 applies the 80/20 rule to individual readings, SSPC-PA 2 checks spot and area averages against the specified range — and only one set can be chosen in a single project.
What is ISO 19840:2012
Full title: “Paints and varnishes — Corrosion protection of steel structures by protective paint systems — Measurement of, and acceptance criteria for, the thickness of dry films on rough surfaces”.
It solves one specific problem: blasted steel has a rough surface (typical roughness Rz 50-100 μm), and ordinary gauges read high. The standard specifies how to compensate roughness, how to sample, and how to judge acceptance.
Key facts
| Item | ISO 19840:2012 | SSPC-PA 2-2022 |
|---|---|---|
| Issuing body | ISO / TC 35 | AMPP (formerly SSPC + NACE) |
| Companion standard | ISO 12944 | ASTM D7091-22 |
| Judgment rule | 80/20 rule: mean ≥ NDFT, every reading ≥ 80 % NDFT, fewer than 20 % of readings between 80 % and 100 % NDFT, none above the specified maximum | No 80/20 rule: each spot ≥ 80 % of specified min and ≤ 120 % of specified max (Level 3); area average within specified range |
| Sampling | Individual readings per sampling plan by inspection area; not applicable below 40 µm NDFT | 5 spots per ~10 m² (100 ft²) area; ≤30 m² every area, 30–100 m² three areas, then +1 area per extra 100 m² |
| Roughness treatment | Deducts a correction value for the blast profile | Requires BMR correction |
| Thickness cap | Sets a maximum thickness cap | Each spot ≤ 120 % of specified max (Level 3); area average within specified range |
| Region | European / international projects | US painting work |
Why roughness must be corrected
Blasted-surface peaks can raise instrument readings. SSPC-PA 2 follows ASTM D7091 for base-metal reading (BMR) compensation or appropriate Type 2 adjustment on an uncoated substrate of the same roughness. ISO 19840 instead deducts the correction value determined for the blast profile from each individual reading. Apply the procedure of the standard specified by the contract.
The place of ISO 12944
ISO 12944 is a complete set of specifications for anti-corrosion paint systems for steel structures (9 parts in total); ISO 19840 is its thickness-acceptance companion. ISO 12944 Part 2 defines atmospheric corrosivity categories C1 (very low) to C5 (very high industrial/marine) and the immersion/soil categories Im1-Im3; Part 5 specifies the coating system selection; Part 8 covers acceptance and references ISO 19840.
Practical selection advice
- The contract explicitly designates a standard → follow the contract; do not “add” the other set in the report;
- International projects → it is recommended to clarify which set to use at the contract negotiation stage and write it into the technical specification;
- Multiple parties inspecting one project → when each party uses its own instrument, the instrument differences should be compared and recorded in advance.
Corresponding models in the Dongru product line
Rough-surface coatings are usually thick; the corresponding Dongru models are DR280A (0-1250 μm, Fe/NFe dual mode), DR380A (0-2000 μm), DR5000S (0-5000 μm) and DR6000A (0-6000 μm). All of these support substrate zero adjustment and can perform BMR correction.
Common misconceptions
- ❌ “The two standards can substitute for each other” — their sampling plans and acceptance rules differ (the 80/20 rule belongs to ISO 19840 only); mixing causes disputes.
- ❌ “Roughness correction can be made up later with an average” — correction must be measured on bare substrate of the same roughness, not calculated afterwards.
- ❌ “ISO 19840 only applies to European projects” — it is an international standard and can be referenced by any project using the ISO system.
Related instruments: DONGRU DR-A5 Automotive Paint Thickness Gauge · Request a quote