Coating and Lining Material Types and Selection
What the exam expects
You need a working map of generic coating families and where each fits. Epoxies (amine or polyamide cured) give good chemical resistance and adhesion but chalk under UV; polyurethanes offer UV stability and color retention; zinc-rich primers provide galvanic protection; inorganic zinc resists high temperature and solvents but needs a suitable tie coat; phenolics/novolacs handle aggressive acids; silicones handle high temperature; coal tar epoxies resist water immersion but are increasingly restricted. Know glass-flake and high-build variants for immersion service.
Selection is driven by: service environment (atmospheric, immersion, buried, chemical exposure), temperature, expected life, substrate, and applicable standards. Understand DFT vs. WFT and volume solids relationships, plus the concept of pot life and induction time.
Traps
- Choosing an epoxy for a UV-exposed finish coat; epoxies chalk and lose gloss.
- Confusing zinc-rich primers with inhibitive primers - different mechanisms.
- Forgetting that inorganic zinc requires a mist coat or tie coat to avoid pinholing in the topcoat.
- Ignoring that high-volume-solids products can still require multiple coats to reach specified DFT.
Rule of thumb
WFT = DFT ÷ volume solids (fraction). For a 100 µm DFT with a 65% volume-solids product, WFT ≈ 154 µm.
Worked example
A tank interior in dilute acid service at ambient temperature: a novolac epoxy or glass-flake lining is a reasonable selection; a standard amine epoxy topcoat exposed to sunlight on the exterior would be unsuitable without a UV-stable finish.