I am working on my 46 truck. My paint guy explained it to me this way - Acrylic Enamel is like asphalt and urethane is like cement harder and more durable. Both enamel and urethane are available with catylist. Mine body color is a single stage urethane catylist product. What I discovered is the urethane is much harder to apply without runs. You have apply lighter coats and wait a few minutes between them. I sprayed my engine with acrylic enamel. It was so much eaiser. The best product now is the epoxy primer. You can sandblast or strip the metal then epoxy primer the metal then do the body work. This is great for those of us doing this in your spare time. To get the urethane to really shine and look like glass you have to wet sand and buff it . It looks great when you do.