How the test works
A dolly is bonded to the cured coating with a suitable adhesive. Once the adhesive has cured, a portable tester applies an increasing tensile load normal to the surface until the dolly detaches or a specified load is reached. The tester reports the pull-off stress; the inspector then examines both the dolly face and the test spot to record where the break occurred. The test is destructive: every test spot must be repaired to the coating specification afterwards.
Which standard applies
| Standard | Substrate | Scope |
|---|---|---|
| ASTM D4541 | Metal | Pull-off strength of coatings with portable adhesion testers |
| ISO 4624 | Rigid substrates, steel included | Pull-off test for paints, varnishes and coating systems |
| ASTM D7234 | Concrete | Pull-off adhesion of coatings on concrete with portable testers |
ASTM D4541 recognizes several types of tester and warns that results from different instruments are not directly interchangeable. A specification should therefore name the standard, the tester type, the dolly size, whether the coating is cut around the dolly before pulling, and the acceptance value — and those values should be read against the coating manufacturer's data sheet rather than a generic figure.
Reading the failure mode
The number alone says little; the location of the break carries the information.
- Adhesive failure — separation at an interface, either between substrate and first coat or between two coats. Substrate-to-primer failure points to surface preparation: inadequate surface profile, contamination or flash rust. Intercoat failure points to an exceeded recoat window or a contaminated layer.
- Cohesive failure — the break runs through one layer, leaving the same material on dolly and test spot. Adhesion exceeded the strength of that layer. On concrete under ASTM D7234, cohesive failure in the concrete itself is common and indicates the strength of the substrate surface, not a coating defect.
- Glue failure — the dolly adhesive let go first. The coating held at least the recorded stress, so the value is a lower bound only and the test does not identify a weak point.
ISO 4624 codes each failure location with letters and reports the percentage of area for each, which makes mixed failures traceable between inspectors.
Where it fits in coating QA
Pull-off testing complements, rather than replaces, dry film thickness and holiday testing. It is used on trial patches, on procedure qualification panels, and in failure investigation, where the failure mode often explains a premature lining failure faster than the stress value does.