Adhesion testing is destructive, so it happens on sacrificial areas or is repaired afterwards. That cost buys real information about whether a system will stay attached, provided the result is interpreted correctly.

Pull off testing

A dolly is bonded to the coating surface with adhesive, the coating around it is scored to isolate the test area, and a calibrated instrument pulls the dolly perpendicular to the surface until something breaks. The result is a stress value, commonly expressed in megapascals.

The number matters, but the failure mode matters more. Adhesive failure at the substrate interface indicates a preparation problem. Cohesive failure within a coat means that coat is the weak link but bonding is sound. Failure at an interface between coats points to an overcoating problem, often a missed recoat window as described in our recoat interval guide. Glue failure means the test itself is invalid and must be repeated.

Cross hatch testing

A pattern of cuts is made through the coating to the substrate, forming a grid, then tape is applied and removed. The proportion of squares detached is rated against a classification scale. It is quick, cheap and suitable for thin coatings, typically under about 250 microns, and is used widely on decorative and light industrial work.

It is comparative rather than quantitative. It tells you whether adhesion is acceptable or poor, not the stress the bond can carry, which is why heavy duty and immersion specifications call for pull off testing instead.

Practical considerations

Test only after full cure, since testing early gives artificially low results that trigger unnecessary investigation. Prepare the dolly and coating surface properly, as poor adhesive preparation is the most common source of invalid pull off results. Test in representative areas rather than convenient ones, and record location, temperature and cure age with the result.

What to do with a poor result

Investigate the failure mode before deciding action. Interface failure at the substrate points back to preparation records: profile, salts, dust, all covered in our hold points article. Widespread interface failure between coats usually means removal and reapplication of the affected coats rather than local repair.

MENA Industrial Supplies supplies coatings with published adhesion data and preparation requirements on every technical data sheet.