A blasted steel profile has peaks and valleys. Primer must cover that surface while the thickness gauge reports a value under its own calibration and measurement definition. One roughness value and one film average cannot describe every local coverage path. [1]
Put profile and coating on the same section
Sketch the steel profile, primer surface and any later layers. Identify exposed or thinly covered peak regions and retained coating in valleys. Record whether the reported thickness uses a rough-substrate correction or a different reference permitted by the specified method.
| Quantity | Evidence | Question left open |
|---|---|---|
| Profile parameter | Qualified profile measurement and sampling | Peak shape and local distribution |
| Dry-film thickness | Gauge/probe and substrate basis | Local continuity over each peak |
| Cross-section or representative coupon | Actual coating/profile image | How well it represents the tank geometry |
| Local defect observation | Accessible inspection under approved method | Regions masked by geometry or later coats |
Do not subtract arbitrary roughness from film thickness
A roughness parameter is not automatically a coating-thickness correction. Peak-to-valley depth, average roughness and the gauge’s effective reading describe different quantities. Use the actual inspection method and instrument guidance to reconcile them.
Compare the profile with the coating system’s specified preparation range. Excessive or inconsistent profile can change the coverage burden. Applying more paint without checking the system limits can introduce another problem rather than resolve the original geometry.
Preparation must address the actual substrate and contamination before coating; surface identity and the accepted preparation state belong in the repair record. [2]
Separate profile height from gauge thickness
Assumptions: an approved local peak-coverage minimum is 40 μm. A mapped section uses the valley as z = 0, locates a steel peak at z = 60 μm and the coating surface directly above it at z = 90 μm. Local coverage is 90 − 60 = 30 μm, deficient by 10 μm: hold that region for the approved repair. A separate gauge result of 100 μm cannot be substituted for z = 90 μm. Subtracting roughness from a gauge reading without a common datum would miss the demonstrated deficiency.
References
- ISO, ISO 2808:2019, Paints and varnishes — Determination of film thickness.
- ISO, ISO 12944-4:2017, Paints and varnishes — Corrosion protection of steel structures by protective paint systems — Part 4: Types of surface and surface preparation.