Why are you undercoating the car
You live in Arizona for cripes sake
If you are'nt going to drive in snow/salt/mud you should be fine just painting everything.
I live in Missouri, middle of the rust belt. No undercoat on my car. The bottom is painted black but a lot of guys around here with showcars paint the underneath bodycolor and it's as nice as the top.
Your car was originally primed only underneath and the dealer installed any undercoat. If I'm not wrong, Show judges frown bigtime on undercoating.