Blast Profile Too Deep on Thin Plate
A crew is blasting the shell plating of a small coastal vessel. The specification calls for Sa 2.5 with an angular profile of 50–75 µm, measured by replica tape. The plate in one area is 6 mm thick and shows visible pitting from prior service. The blaster, aiming to remove all pits, increases nozzle pressure and dwell time. Replica tape readings in that area come back at 95–110 µm. The inspector also notes the plate is warm to the touch and a slight dishing is visible under raking light. The coating system is a zinc-rich primer followed by epoxy and a topcoat.
1. Evaluate the profile readings against the specification and explain the consequences of over-blasting for this coating system.
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The readings of 95–110 µm exceed the specified 50–75 µm upper limit by a wide margin - this is over-blasting, not a marginal miss. The blaster's intent (removing pits) is understandable but the method is wrong.
Consequences specific to a zinc-rich primer system:
- Excessive profile creates deep valleys that a zinc-rich primer may not fully bridge at its specified DFT. Uncoated peaks and valleys become sites for early rusting and pinpoint corrosion.
- Zinc-rich primers are particularly sensitive to profile: too rough a surface can leave peaks protruding through the primer, and the high surface area of a deep angular profile consumes zinc faster, reducing cathodic protection life.
- Thin plate risk: 6 mm plate with visible dishing under raking light and a warm surface suggests the blaster may be deforming or thinning the plate. That is a structural concern, not just a coating one.
- Pitting removal by blasting is the wrong approach - deep pits should be evaluated for weld repair or filling; blasting to remove them drives the whole surface past the profile limit.
The area fails the profile specification and must be addressed before priming.
2. What should the inspector do next, and how should the pitting be handled without repeating the over-blast?
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Sequence of actions:
- Stop the blasting in that area and document: profile readings (95–110 µm), locations, plate thickness, dishing observation, and the warm surface.
- Assess plate integrity. Visible dishing on 6 mm plate under raking light is a red flag for mechanical damage. Recommend ultrasonic thickness measurement or a surveyor's assessment before any further work. If the plate is thinned or deformed, coating is secondary to structural repair.
- Address the profile. Over-blasted profile cannot be 'smoothed' by more blasting. Options: accept only if the coating data sheet permits a higher profile (it does not here - 50–75 µm is specified), or apply a suitable primer/sealer designed to bridge a rough profile, or in severe cases replace/repair the plate. Do not simply prime over 110 µm profile with a standard zinc-rich primer.
- Handle the pits correctly. Deep pits should be evaluated individually - weld repair, pit filler, or acceptance as-is if within structural limits. Blasting should target the specified profile, not pit elimination.
- Correct the blaster's parameters for the remaining area: reduce pressure and dwell time, use the correct abrasive, and verify profile with replica tape at the start of each area before full coverage.
Document the corrective actions and re-inspect before primer application.