Coating Types, Selection, and Specification
What the candidate must know
Know the major generic coating families and their service niches: epoxies (good chemical/immersion resistance, chalk in UV), polyurethanes (UV-stable topcoats), zinc-rich primers (galvanic protection, inorganic vs organic), alkyds (economical, limited chemical resistance), and high-performance systems for immersion or severe environments. Understand the role of a coating system: primer for adhesion and corrosion protection, intermediate for barrier, topcoat for environment and appearance.
Selection depends on environment (atmospheric category, immersion, chemical exposure), substrate, service temperature, and expected life. Read the product data sheet for volume solids, recommended DFT per coat, recoat windows, and surface prep requirement. Specifications should be unambiguous: state the standard, the grade, the DFT range, and the test method.
Traps in questions
- Choosing a UV-sensitive epoxy as an exterior topcoat.
- Assuming a zinc-rich primer is a stand-alone finish rather than a system component.
- Ignoring service temperature limits (some coatings degrade above a threshold).
- Treating generic family names as interchangeable; formulations differ.
Rule of thumb / worked example
Match the system to the exposure. For an atmospheric exterior steel structure with moderate corrosivity, a typical system might be a zinc-rich primer, an epoxy intermediate, and a UV-stable polyurethane topcoat. For continuous immersion, avoid UV-dependent topcoats and select an epoxy or equivalent rated for immersion at the stated temperature. Always verify each layer's data sheet supports the intended service, rather than assuming the family name guarantees performance.