It doesn't need to harden, just before it hardens is my suggestion. If the car is out of the sun and it was in the sun beforehand the surface may still be hot this will bake it on there.
I've always used Zymol. I love that stuff. I get the $20 bottle, but if you like good stuff check this out:
http://www.eshine.ca/zymol_vintage_glaze_22oz-2320.php