first off, I paint 1:1 cars for a living and have been for many years. urethanes are great products and I wouldnt use anything else.
please buy the intended reducer for the clear you are using, I know that other stuff works but is it worth taking a chance ruining something you spent all that time and money on for the cheapest component in the process?
shopline is a great clear, cheap & easy to use, and looks great. I highly recomend it.
clean up using lacquer thinner for airbrush or spray guns