is single stage urethane considered a synthetic enamel? redface.gif

No. There are many types of enamel, sythetic being one of them. hardener CAN be added to synthetic enamel. But it is not urethane. Automotive urethane paints MUST use a hardener and is much better paint than enamel.
They call it 'single stage' due to it's singular application without the need for a clear coat. Single stage has the 'gloss' added to it.

thanks Jimmy P!