Coating Types, Formulation, and Selection
Resins and cure mechanisms
Know the generic families and how they fail. Epoxies (amine- or polyamide-cured, two-component) give excellent chemical and barrier performance but chalk under UV, so they are usually topcoated. Aliphatic polyurethanes and polysiloxanes hold color and gloss in sunlight. Zinc-rich primers protect steel sacrificially; organic zinc relies on the binder, while inorganic (silicate) zinc cures by reacting with atmospheric moisture and needs humidity to develop properties. Alkyds cure by oxidation and are attacked by alkali, so they are poor on fresh concrete or in immersion. Coal tar epoxies perform in immersion but degrade in sunlight. Silicones serve high-temperature service.
Formulation vocabulary
Pigment volume concentration (PVC), critical PVC (CPVC), volume solids, volatile organic content (VOC), and pigment-to-binder ratio. The single most testable idea: below CPVC the binder fully encapsulates pigment and the film is a coherent barrier; above CPVC there is insufficient binder, and the film is porous and permeable. Volume solids is the number that converts wet film to dry film and drives VOC and coverage calculations.
Selection logic
Match primer to substrate (zinc for steel, tie coats for galvanizing, penetrating or masonry coatings for concrete), topcoat to environment (UV, chemical, immersion, abrasion, temperature), and the whole system to the surface preparation that is actually achievable on site. Confirm intercoat compatibility and recoat windows.
Worked example
A tank interior calls for an epoxy; the exterior of the same tank is in direct sun. An epoxy topcoat will chalk and fade though corrosion protection may persist - the question is appearance, not failure. Specify a UV-stable topcoat and state why.