Quality control: how to measure a surface treatment
From DFT meter to salt spray test — documentation is part of the craftsmanship
In industry a surface treatment is rarely finished just because the paint is dry. Many customers — particularly in offshore, industry and infrastructure projects — require documentation that the work meets the agreed requirements. Quality control is therefore a fixed part of the experienced surface treatments worker's work process, not something accomplished by visual assessment alone.
§Layer thickness — DFT measurement
DFT stands for dry film thickness that is the dry layer thickness after curing. On steel it is typically measured with equipment based on magnetic induction while the eddy current method is used on non-magnetic substrates like aluminum. Layer thickness should be measured at several points on the same surface because it can vary across the workpiece — a single measurement point does not give an accurate picture.
§Adhesion — cross-section test
The cross-cut test according to ISO 2409 assesses how well the coating adheres to the substrate. A grid pattern is cut through the coating down to the substrate, often supplemented by a tape test where you tear tape off the cut and assess how much coating is released. The result is typically rated on a scale from 0 (no peeling) to 5 (severe peeling).
§Gloss and appearance
A glazier who can assess both options can give the customer a real and informed choice between preserving and renewing.
§Porosity — holiday test
On thick protective coatings, such as those found on pipelines and tanks, a holiday test (also called spark test) is used. Here, current is applied over the surface to reveal pores and pinholes which otherwise could allow corrosive media access to the steel under the coating.
§Accelerated corrosion testing — salt spray
ISO 9227 describes salt spray tests, including the neutral salt spray variant (NSS), an acetic acid variant (AASS) and a copper-accelerated variant (CASS). In a test chamber the coating is exposed to an aggressive, salt-containing environment over an accelerated period, and the result is often used to document that an entire coating system meets a given ISO 12944 corrosion category.
| Inspection | What it shows | Typically used for |
|---|---|---|
| DFT measurement | Layer thickness | All types of coatings |
| Cross-cut test (ISO 2409) | Adhesion to the base | Sample inspection on new tasks |
| Gloss measurement | Surface appearance/finish | Visible, decorative surfaces |
| Holiday test | Porosity/pinholes | Thick protective coatings such as tanks and pipes |
| Salt spray test (ISO 9227) | Corrosion resistance over time | Approval of entire coating systems |
Common to all methods is that the measuring equipment must be calibrated, and that the results only make sense if they are held against the specific requirement from product data sheet or agreed standard. Quality control is not an after-check of the work — it is part of it.