Get Sherwins williams heavy primer, you won't sand through, if the body is close to straight. If not, go back and fill in the low spots in 15 minutes. They primer really shows up the lows.
We are not starting with bare metal first off, if this helps. I also already have Transtar Euro primer Low Voc. How many coats should I spray before letting it dry and going first round of sanding? aka One coat over original paint then flash then another then dry?