_ Both urethane and epoxy are polymers. That means they are large molecule substances that have a repeating structure. Epoxy is a blend of hardener and resin made from polyamine and epoxide. Urethane is a composition of carbon-based, or organic links that react with a chemically bonding element known as a monomer.
_ Many industrial products use epoxy and urethane. Resins, foam, adhesive agents, paints, and curing agents are examples of products that use them. They have similar properties. The differences between the two are key factors that go into the preferences each industry has. Texture, flexibility, chemical resistance, and color are considerations industries use to decide which to use. In terms of price, there is no significant difference.
Color
_ The paint industry wants paint that does not change color. Industries using resin require substances that remain transparent. Ultraviolet light causes epoxy to yellow after a few months or years. Urethane is resistant to UV light and stays the same color longer than epoxy.
Chemical Resistance
_ Nearly all industries using urethane or epoxy products want the substance to be chemical resistant. Corrosion or acid or base reaction needs to be prevented. Acids and bases do not readily interact with epoxy. It is highly chemical resistant. Acids and bases must be diluted to avoid interaction with urethane. Urethane wears faster than epoxy.
Flexibility
_ Epoxy and urethane are used by foam and adhesive industries to bond surfaces. Firmness and flexibility are important. Of the two, epoxy is a higher-strength substance that prevents a surface from moving. Though epoxy is strong, it will crack on surfaces that frequently move. Urethane retains flexibility. Structures that contract or expand with seasons or temperatures are better suited for urethane.
Texture
_ Dependence on the need for a chalky or smooth substance determines an industry’s choice between urethane and epoxy. When it ages, epoxy becomes chalky and brittle to the touch. Urethane remains smooth. It is also better for areas or surfaces exposed to water.
Primer for bodypainting
_ Urethane is the most common auto body primer. The consistency fills well and is easy to sand. When properly applied, it yields a perfect body. For surfacing and plastic body filler, it is the most all around primer. 
Primer for bare metal
_ Epoxy is a corrosion fighter. The sticky resin contained in epoxy provides good adhesion to most substances. It is ideal as a bare metal primer/sealer when no surfacing is required. It can be used as a non-sanding primer and painted over.