Measuring Sealers for Metallised Coatings

While the column focuses on measuring the DFT or WFT of the sealer, it also defines the metalizing sealer, and draws the distinction between the sealer and a paint topcoat that may be applied over a sealed metalized coating.

“Sealers for thermal spray coatings (TSC) are low-viscosity, clear or pigmented paints (including lacquers and vinyls) formulated to flow over and be absorbed into the natural pores of the TSC.” On the other hand, “A topcoat is essentially a full coat of paint and may be applied over a seal coat.”

Industry standards make it clear that a paint topcoat should not be applied directly to a metalized coating. Yet many specification writers, those who recommend materials, and even some metalizing companies do not understand or follow the basic principal – SEAL FIRST.

Lets re-state the common understanding on when topcoats are used.

1. Decorative finish is required (e.g., a bridge fascia girder).
2. The environment includes frequent salt or fresh water (excluding potable water) spray, splash, or immersion service.
3. The environment is very acidic or very alkaline.
4. The metallic coating is subject to direct attack by specific chemicals.
5. Additional abrasion resistance is required.

We would add to this the comment that life-to-first-maintenance and maintenance interval are affected by sealing and topcoating. These factors should be considered when choosing to topcoat or not.

Reports on the subject are available.

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