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ISO 28199-3:2021 — Assessment of Sagging, Bubble Formation, Pinholing and Hiding Power in Spray-Applied Coatings
Paints and varnishes — Evaluation of coating properties related to spray application — Part 3
Introduction to ISO 28199-3:2021
ISO 28199-3:2021 completes the evaluation framework for spray-applied coating systems by addressing four common yet critical defects: sagging (runs), bubble formation, pinholing, and hiding power. While ISO 28199-2 focuses on colour-related properties, this part targets structural coating defects that directly impact appearance, corrosion protection, and production yield. The standard provides both visual assessment methods and, new in this edition, quantitative measurement techniques for all evaluations.
Sagging and pinholing are often competing defects in paint formulation — a paint designed to resist sagging may be more prone to pinholing at higher film builds. ISO 28199-3 provides the objective framework to balance these trade-offs.
Sagging Tendency: Measurement and Interpretation
Sagging tendency is evaluated using a specially perforated test panel prepared per ISO 28199-1 Version A. After drying/curing, the longest sag extending from the lower edge of each hole is measured. The critical parameter is the distance from the hole’s lower edge to the 10 mm sag point, correlated with the corresponding film thickness. A control chart of film thickness versus sag length enables quantitative comparison between coating formulations. Greater distance at a given film thickness indicates superior sag resistance.
| Defect Type | Test Specimen | Evaluation Method | Key Limit Parameter |
|---|---|---|---|
| Sagging | Perforated panel, wedge coating | Visual/measured sag length from hole edge | Film thickness at 10 mm sag length |
| Bubbles | Wedge-coated panel | Visual + 3D profilometry | Bubble formation limit film thickness |
| Pinholing | Wedge-coated panel | Visual + 3D profilometry | Pinholing limit film thickness |
| Hiding power | Contrast strip with wedge coating | Contrast ratio per ISO 6504-3 | Film thickness at contrast ratio = 0.98 |
Bubble Formation and Pinholing
Bubble formation and pinholing are assessed after drying/curing on wedge-coated panels. The bubble formation limit is defined as the film thickness at which the number of bubbles first exceeds the agreed threshold (a single bubble does not define the limit, and edge/perforation-area bubbles are excluded). The 2021 edition added three-dimensional photographs and cross-sectional profiles for both bubbles and pinholes, enabling more precise morphological classification. Cratering — often confused with pinholing — is also defined and illustrated, with examples showing the characteristic circular depressions caused by surface tension irregularities from contamination.
Pinholing and cratering are frequently misidentified in production. Pinholes penetrate through the coating thickness as narrow channels, while craters are shallow circular depressions that may not reach the substrate. ISO 28199-3 provides clear morphological criteria for differentiation.
Hiding Power and Engineering Applications
Hiding power assessment distinguishes between “process hiding power” (evaluated per ISO 28199-2) and “black/white hiding power” measured per ISO 6504-3 using a contrast substrate. This dual approach ensures that both the process-specific coverage and the fundamental optical hiding capability are characterized. For production engineers, the standard’s practical value lies in establishing maximum allowable film thickness limits for each defect type, enabling the construction of comprehensive process windows that simultaneously satisfy all quality requirements.
A complete coating process window should define four critical film thickness limits: minimum for hiding power, minimum for colour stability (from ISO 28199-2), maximum before sagging onset, and maximum before bubble formation. The usable process window lies between these boundaries.
Engineering Design Insights
The 2021 revision of ISO 28199-3 represents a significant advancement by incorporating measurement-based evaluation alongside traditional visual assessment. This enables automated production-line inspection and reduces inter-operator variability. Key design considerations include ensuring proper test panel geometry with defined perforation patterns, controlling drying/curing conditions within specified tolerances, and using calibrated film thickness measurement patterns per ISO 28199-1. The standard’s annexes with 3D defect profiles serve as valuable reference for training quality control personnel.
Q1: What is the difference between sagging and cratering?
A: Sagging is downward flow of the coating under gravity, appearing as runs or curtains. Cratering is the formation of small circular depressions caused by localized surface tension differences, typically from contamination.
A: Sagging is downward flow of the coating under gravity, appearing as runs or curtains. Cratering is the formation of small circular depressions caused by localized surface tension differences, typically from contamination.
Q2: Can bubble formation and pinholing be evaluated simultaneously?
A: Yes, both can be assessed on the same wedge-coated panel after drying/curing, using the film thickness measurement pattern specified in ISO 28199-1.
A: Yes, both can be assessed on the same wedge-coated panel after drying/curing, using the film thickness measurement pattern specified in ISO 28199-1.
Q3: What is the recommended test panel material?
A: Steel panels with a defined surface profile, prepared per ISO 28199-1:2021, typically using phosphated cold-rolled steel with a controlled surface roughness.
A: Steel panels with a defined surface profile, prepared per ISO 28199-1:2021, typically using phosphated cold-rolled steel with a controlled surface roughness.
Q4: How does the 2021 edition differ from the 2009 edition?
A: Key changes include the addition of measuring techniques for all evaluations, 3-D photographs of defects, updated normative references, and the relocation of bubble formation limit and cratering definitions from ISO 28199-1.
A: Key changes include the addition of measuring techniques for all evaluations, 3-D photographs of defects, updated normative references, and the relocation of bubble formation limit and cratering definitions from ISO 28199-1.
