painting question

Technically, the bare metal areas would need a metal etching primer for the primer to adhere properly. But the question about grit is a question about how pristine are you wanting the truck in the end. If this is merely a repaint of an average daily driver, 120 thoroughly over the entire truck followed by at least 2 coats of FILLER primer would suffice, allowing it to cure for at least a full 24 hours before sanding.
If it is more of a show car job you are looking for, 22o is a minimum before primer. Then the etching primer followed by a minimum of 2 coats of good quality primer and a guide coat dust to finish. Then block sand starting with 320-400 to remove the high spots and recoat with primer and guide coat and so on till the guide coat comes off smooth over the entire vehicle. Wet sanding afterward with 400, 800, etc. is optional depending on just how perfect this needs to be.