CSLB C-33 Painting & DecoratingSurface PreparationMedium
A painting contractor is preparing to paint a new, unprimed galvanized metal ductwork. Which type of primer is specifically recommended for this substrate to ensure proper adhesion and prevent premature coating failure?
- AZinc-rich primer.
- BWater-based latex primer.
- CShellac-based primer.
- DOil-based alkyd primer.
Show answer & explanationAnswer & explanation
Correct answer: A. Zinc-rich primer.
New galvanized metal requires a specialized primer, typically a zinc-rich primer or a specific DTM (Direct-to-Metal) acrylic primer designed for galvanized surfaces. The zinc reacts with the zinc coating on the galvanized metal, providing excellent adhesion and corrosion protection. Standard oil-based primers can react with the zinc, leading to poor adhesion and peeling, while standard latex and shellac primers lack the necessary properties for this substrate.
Why the other options are wrong
- B. Standard water-based latex primers may not adhere well to galvanized metal without specialized formulations.
- C. Shellac-based primers are primarily used for stain blocking and adhesion to challenging interior surfaces, not typically for galvanized metal.
- D. Oil-based alkyd primers can react with the zinc in galvanized metal, leading to saponification, poor adhesion, and peeling.
Galvanized Metal Priming
Galvanized metal requires specialized primers, such as zinc-rich or DTM acrylics, to ensure proper adhesion and prevent chemical reactions that lead to coating failure.
- Galvanized surfaces are reactive.
- Zinc-rich primers offer cathodic protection.
- Avoid standard alkyd primers on new galvanized.
Memory trick: Galvanized needs Zinc's zeal, not just basic peel-proof deals.