ANY paint that comes in a can is an enamel, which means it uses the evaporation of the base solvents to facilitate in the curing process. Real automotive Urathane uses a two part mixing process. A resin, and a hardener which catalyzes a chemical process that causes it to harden, along with the evaporation of solvents to facilitate curing.
Two part automotive urathane is the best way to achieve a proper finish that is hard enough for general use on a bicycle. Enamels will almost always chip and flake with wear and tear.

If you don't have painting equipment, ie a spray gun and air compressor, then the local paint shop can pre-mix clear coat into a spray can for you to use within 24 hours of mixing. After that, it will harden in the can, and be useless.