I think the answer is, the "base Coat" is nothing more than a "colored primer" after which a 2 part urethane clear MUST be applied to finish the process.

However, there are 2 part systems such as DuPont System-2000 which is a "single" step application of a 2 part "colored" urethane which has a gloss finish.

BOTH are fuel proof to Kero and Gasoline.