What is Powder Coating?
MIL-DTL-53039

Powder coating is a system that applies, to a substrate, finely ground charged particles of pigments & resins.  This powder is heated to above its melting point causing the powder to flow & fuse, creating a continuous film.  The result is a uniform, durable, high-quality, and attractive finish.  Powder Coating is also [used] as a general term and usually non-specific, to extol the virtues of a durable painted finish on metal surfaces.  Below are some powder coating specifics.

Epoxy Powder Coatings

Epoxy based powder coatings were the first thermosetting system to become commercially available and are still widely used today. Epoxy coatings are used primarily as functional coatings for substrate protection where inherent toughness, corrosion resistance, flexibility, and adhesion are required. The primary limitation of epoxy-based coatings is poor weatherability. 
Typical applications include pipe coatings, electric meters, metal furniture, shelving, electrical parts, refrigerator liners, dryer drums, and high voltage switchgear

Hybrid Powder Coatings

Hybrid coatings share many characteristics with standard epoxy-based powder coatings.  The primary differences are improved resistance to overbake yellowing during cure, somewhat softer films, and slightly improved weatherability.  Like epoxies, hybrid coatings are not considered suitable for exterior applications, even though their deterioration and discoloration when exposed to sunlight is slower then pure epoxies.  Hybrid coatings have similar characteristics associated with polyester formulations in terms of impact and bend resistance.  Hybrid coatings are often used in the same applications as epoxies particularly when slight improvement in weathering and overbake stability is required. 
Typical applications are shelving, hot water heaters, office furniture, power tools, tool boxes, electrical boxes, fire extinguishers, under-hood automotive and oil filters.

Polyurethane Powder Coating

Urethane-cured polyester powder coatings combine very smooth, exceptional thin film capability, with excellent mar and chip resistance, and good weathering properties.  Adhesion to properly prepared (pretreated) substrates will provide very durable coatings with long-term resistance to humidity and corrosion.  Polyurethanes are typically resistant to many diluted aqueous acids, salts, aromatic and aliphatic hydrocarbons, grease, and oils. 
Typical applications utilizing polyurethane coatings include fluorescent light fixtures, lawn and garden equipment, electrical enclosures, playground equipment, range panels, air conditioners, automotive trim, and patio furniture.

TGIC Powder Coatings
(Triglycidylisocyanurate)