Aircraft paint needs to handle harsh weather conditions, bend and flex, withstand different chemicals and still look good. Because of these rigorous demands, there are generally only two types of paint used for aircraft: epoxy and enamel, one of which is less hazardous but less durable and the other which is stronger but more expensive.
Epoxy is a polyurethane paint formed by the reaction of a hardener and a resin. Epoxies are known for good adhesion to surfaces, high heat and chemical resistance and very good electrical insulation, all of which make it better than other paints, such as lacquer, for use with aircraft.
Advantages of Epoxy
Because epoxy holds well to surfaces and doesn't dry as hard as enamel, epoxy doesn't become brittle and crack or chip from the plane. Even when mixed with plasticizers, enamel doesn't have the same flexible properties as epoxy. Similarly, epoxy has a higher resistance to chemicals, meaning it won't break down when exposed, nor does it fade or oxidize as quickly.
Industrial enamel is very different from commercial enamel. For aircraft use, enamel is extremely hard and resistant to chemicals and conditions. It also keeps a tremendous shine even after exposure to hard conditions. The toughness of enamel makes it strong enough to handle the conditions to which an aircraft is regularly exposed. However, its hardness also keeps it from bending as the aircraft requires.
Advantages of Enamel
While not as resistant or flexible as epoxy, industrial enamel has two major advantages: cost and health. Polyurethane (epoxy) paints give off cyanide gas when sprayed; thus workers who apply epoxy must be extremely cautions to avoid exposure. As well, enamel paints are considerably cheaper than epoxy. Often the two paints are split by laying a base coat of enamel to provide the color and design for the aircraft and then applying a second coat of clear polyurethane to supply strength and extra shine.