The acrylic enamel can be sprayed without a catalyst, the hardener with help with gloss retention, paint durability, and negates the re-coat time if you have to re-spray a spot. As far as I know, the acrylic urethane has to have the hardener in order for the finish to cure. If I'm spraying either one of those, I use the hardener.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy