Coating Systems: Selection, Application, and Failure
What you must know
Coatings are barrier and, for some systems, sacrificial or inhibitive protection. Know the components: resin/binder, pigment, solvent, and additives, and how each affects performance. Understand surface preparation as the dominant variable: SSPC/NACE surface preparation standards (solvent cleaning, hand/power tool, abrasive blast grades such as near-white and white metal), anchor profile, and the effect of mill scale, salts, and moisture on adhesion. Most coating failures trace to poor surface preparation or application conditions (dew point, humidity, temperature).
Know generic coating families and where they fit: epoxies (good chemical resistance, poor UV), polyurethanes (UV-stable topcoats), zinc-rich primers (sacrificial/galvanic protection), coal-tar and asphaltic systems (legacy), and high-performance systems such as glass-flake and novolac epoxies. Understand the role of a zinc-rich primer in a three-coat system and how it protects at holidays.
Understand failure modes: blistering (osmotic vs. chemical), undercutting, cathodic disbondment (especially on cathodically protected structures), chalking, cracking, and delamination. Know that a coating defect on a cathodically protected structure can suffer cathodic disbondment from the alkaline environment generated at the cathode.
Traps
Questions often present a coating failure and ask for the most likely cause; surface preparation and moisture during application are frequent answers. Another trap: assuming a coating is a substitute for cathodic protection rather than a complement.
Rule of thumb
Dew point: do not apply coatings when the substrate temperature is within about 3°C (5°F) of the dew point, and follow the manufacturer's recoat window.