all of this sounds very poor..
polyeurethane is a varnish
http://en.wikipedia.org/wiki/Polyurethane#Varnish
urethane is something else entirely
http://en.wikipedia.org/wiki/Ethyl_carbamate#Uses
enamel is soft...never really cures hard enough(in my experience)and i really don't even know if it is compatible with polyeurethane paint
http://en.wikipedia.org/wiki/Enamel_paint
but why use enamel for color and then top it with polyeurethane?you can get polyeurethane in any color you wish...at any automotive paint store...and then you would KNOW it would never have compatibility issues.
nitro best for clearness?well,i don't know about that either...polyester(which i ASSUME is what you meant by "poly"(poly only is a prefix which means "many")is supposed to be much clearer...but harder to buff and more expensive to apply i think.acrylic is also supposed to be clearer than nitro...
Hey... Sorry if that sounds "poor" to you but that's my personal experience.
Yeah, another option is just buy car paint... NO clear... but avoid duplicolor if you must use car paint. If you're going with spray cans, here in Canada UAP can mix any color for you and put it in a can. Not sure what the product is but I used it once... on a car. Finish was pretty durable.
If going with enamel, like I said, coat it with something hard. If the enamel is thin enough, you should be fine in terms of enamel "softness". Not the best option out there but an option nonetheless.
By Poly, I meant Polyurethane, not polyester. By urethane I meant exactly that: Urethane. A quick google search finds this. I've used this.
Personally, I find that Nitro provides the best "clearness". Personal experience and opinion. For solid colours, I guess its not an issue.
You're right though, spray a finish with a spray gun is the best option if you have the gear and experience. Quick question, how many coats of finish would you spray with a 2-part finish and do you have to sand between coats? I try to avoid finishes where I have to sand between coats. Has anyone heard of catalyzed lacquer?