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What an inspector actually looks at

Reference material, not engineering advice. Every technical claim traces to the cited standard or reference. Technically reviewed August 2026.

Published 2 August 2026 · Core Coatings technical desk

Coating inspection has a reputation for being subjective. It is not. On a properly specified job it is a list of measurements with numbers attached, taken in a fixed order, and most of it happens before a drop of paint is applied.

Below is what that list actually contains. The specific figures are taken from one published government specification for coating structural steelwork, used here as a worked example. Your project specification governs your job — the point is not the numbers, it is the shape of the process and which items are the ones that stop work.

Before anything is blasted

An inspector is checking the steel itself, not the coating.

Sharp edges and weld spatter matter more than they look. Paint pulls away from a sharp arris as it cures, so the thinnest film on the whole fabrication is almost always on an edge that was never dressed. That is why the 2 mm radius is a written requirement and not a suggestion — and why AS/NZS 5131 defines three treatment grades (P1, P2, P3) covering clean-up and dressing of structural steel before corrosion protection.

During and after blasting

The blast standard itself comes from ISO 8501-1, adopted in Australia as AS 1627.9. Sa 2½ is near-white blast cleaning: mill scale, rust and foreign particles removed to the extent that only traces remain as slight stains. AS 1627.4 covers the abrasive blasting process itself and previously defined four classes — Class 1 light, Class 2 medium, Class 2½ near-white and Class 3 white metal.

Cleanliness alone is not enough. The same source is blunt about profile: it should be relatively uniform without rogue peaks. A rogue peak is a single high point that the coating cannot cover to full thickness, and it will show up later as a rust spot in an otherwise sound film.

Conditions on the day

Those readings are taken immediately before or during painting, at least at four-hourly intervals: ambient temperature to ±0.5 °C, steel surface temperature to ±0.5 °C, and relative humidity to ±3 per cent, ambient temperature and humidity measured in a sheltered location next to the area being painted, surface temperature taken on the steelwork itself.

This is the single most commonly skipped item in a workshop, and it is the one that leaves a paper trail. If a coating fails and the daily reports show no readings, the argument is over before it starts.

After the coating goes on

Thickness. Measured to AS 3894.3. On the worked example, the average dry film thickness of each coat must be not less than 80 per cent of the specified nominal DFT and not more than 120 per cent of any specified maximum; the average total system DFT must be not less than 80 per cent of the specified nominal total and not more than 200 per cent of it.

Adhesion. Coatings must be firmly adherent to the substrate and to one another. On the worked example this is confirmed by the X-cut tape test, method A of ASTM D3359, with an average rating of 4A or better and no single result worse than 3A. Areas rejected for adhesion are removed, prepared and recoated. In Australia, AS 3894.9 is the site method for determining adhesion.

Continuity. Holiday or continuity testing is carried out on organic paint coatings that will be buried or immersed, to AS 3894.1 (high voltage brush method) or AS 3894.2 (wet sponge method). Defective areas are marked and their locations recorded for repair.

Visual defects. Each coat and the completed system must be substantially free from runs, sags, wrinkling, fat edges, entrained paint skin, dust or sand, irregular coated patches, dry spray, brush marks, blistering, pinholes and breaks in the coating.

The AS 3894 series, in one place

These are the parts of the AS 3894 family a coating inspector most uses on structural work:

Who carries it

Under AS/NZS 5131 there are two coating quality levels. PC1 applies where coatings go on in corrosivity categories C1 and C2 and surface preparation is by hand or power tool cleaning, or abrasive blast cleaning to Sa 1. PC2 applies to every other corrosivity category and wherever blasting beyond Sa 1 is required — which is most structural work.

PC2 brings a documentation load with it: a quality management system, a quality plan, and an inspection and test plan. And the point that catches fabricators: where the corrosion protection is subcontracted, it remains the fabricator's responsibility, as part of purchasing and subcontracting, to ensure the coating subcontractor complies.

What this means for you

Almost nothing on that list is about paint. It is about edges, cleanliness, salt, humidity, records and thickness — and every one of them is decided before or during application, not after. Failures at inspection are rarely a surprise to anyone who was measuring.

If you have had an inspection go against you in the last twelve months, the Paint Audit maps what was reported back to what it usually points to. Seven questions, and the read comes back on screen.

References

• NZ Transport Agency, S9 Specification for coating steelwork on highway structures, October 2020
• The Australasian Corrosion Association, Steel Surface Preparation Standards
• Steelwork Compliance Australia, Certification and introduction of AS/NZS 5131 — essential changes
• AS 3894 series, Site testing of protective coatings — Building CodeHub entries
• ISO 8501-1:2007, Preparation of steel substrates before application of paints and related products — Visual assessment of surface cleanliness
• ASTM D3359, Standard test methods for rating adhesion by tape test

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