If your using filler primer you can use 180 or 320, primer is thick and will fill 80 scratches unless its spray can primer. If its just to make the truck all one color id 320 it and prime it. Just so you know though primer will soak up water if its left in the elements over time and it will start to rust from under it.
When you decide or if you decide to paint it never finish it with anything greater then 400 or it will peel over time. Only time you will see sand scratches in something that was finished in 400 is if there was not enough base sprayed on from the start.
If you prep right your primer won't peal . You don't even need to sand , if all your doing is covering existing paint all you need is red scuff pads . All you have to do is take tha shine off the existing paint and the primer will stick to it .
He is 1/2 right . Most primers are not a sealer , that is the paints job . But if you get a good primer that also doubles as a sealer you should be ok . I suggest goin to your local keystone/LKQ and picking up some key 3 in one primer , it is really good stuff and is an etching primer as well as a sealer .
Again once you scratch the surface it no longer seals , primer is not as hard as paint .
Even with 400 on some colors you will see sand scratches if your work is not done right . There is a filler coat that nasinn offers that goes on between the color and the clear to fill the scratches that some base coats won't hide .
Your base coat should only be 2 coats . It is there just for pigmentation . Yoir clear you can do several coats and can block / and wet sand that for your shine
