How to Specify Glass-Flake Coating for Seawater Service

Seawater is the most unforgiving environment a coating will ever face: relentless immersion, dissolved oxygen, chlorides that drive under-film corrosion, biological fouling, and mechanical loading from waves, currents and cleaning. A glass-flake coating is engineered to win this fight, but only if the specification is right. Most premature failures are not material failures. They are specification and application failures.

This is the checklist our engineers work to every time we write a seawater specification. Use it to brief your applicator, audit a tender, or sanity-check your own scope.

1Define the service environment first

Before a product is even named, document the conditions the coating must survive:

• Immersion type: permanent immersion, tidal/splash zone, or atmospheric marine.
• Temperature: operating and peak, including any process heat behind the steel.
• Mechanical exposure: abrasion, impact, in-service cleaning method.
• Cathodic protection: is the structure under CP? This affects coating selection and disbondment risk.
• Access for future maintenance: the harder the access, the longer the design life must be.

2Get the surface preparation right: it is 80% of the result

No coating, however advanced, outperforms the surface beneath it. For immersed seawater service the baseline is non-negotiable:

3Specify dry film thickness as a system, not a number

Seawater immersion typically demands a total dry film thickness (DFT) in the 400–1000 micron range, applied in controlled coats. But the number alone is meaningless without:

• Minimum and maximum DFT per coat, since over-application can trap solvent and compromise cure.
• Wet film thickness checks during application to land the DFT first time.
• Inter-coat intervals: minimum and maximum recoat windows, respected on site.

4Control the cure, the step everyone underestimates

A glass-flake vinyl ester only delivers its design properties once fully cured. Immersing a green (under-cured) film is a guaranteed failure. The specification must state:

• Ambient and substrate temperature limits during application and cure.
• Humidity and dew-point control (apply only when the steel is at least 3°C above dew point).
• Minimum cure time, and a confirmation method, before flooding or immersion.

5Build in inspection hold points

Quality is verified, not hoped for. A robust seawater specification names explicit hold points where work stops until inspection passes:

• Post-blast: cleanliness, profile and salt testing.
• During application: WFT and environmental readings logged per coat.
• Post-application: DFT measurement and high-voltage holiday (spark) testing to find pinholes invisible to the eye.
• Pre-service: adhesion testing and cure confirmation.

The specification checklist, in one page

• Service environment documented (immersion, temperature, CP, access).
• Surface prep: SA 2.5, 75–100 µm profile, salts controlled.
• System DFT defined with min/max per coat and recoat windows.
• Environmental and cure limits stated and enforced.
• Hold points and test methods named.
• Documentation and warranty deliverables required in the contract.

Frequently asked questions

What surface preparation is required for glass-flake coating in seawater?

Abrasive blast to SA 2.5 with an angular 75–100 µm profile, with soluble salts controlled to low limits before application.

What dry film thickness is needed?

Typically 400–1000 µm total for immersed seawater service, applied in controlled coats with verified inter-coat timing.

How is the coating tested before service?

DFT measurement, high-voltage holiday testing for pinholes, adhesion testing, and confirmation of full cure before immersion.