Hit the local community college or BOCES and take the auto body classes.
They also take vehicles in as project/teaching items so ask them about
that as well. It is well worth it if you want the paint to look good and
last.

 I hope that the truck is stored inside and that the primer is covering
older paint. If this truck has been stripped and then primed using
conventional primer I would pay to get it painted ASAP. Primer is NOT
WATERPROOF. It will absorb water and hold it next to the steel. Not
good. As for painting it a little at a time, don't bother. It can be
REAL hard to get everything to match if you panel paint it. Instead
Strip the trim and mask it and shoot the entire truck in one shot. If
you want a nice job plan on sanding it down, coating with a good
primer/sealer, sanding some more then blowing it all off and tacking it
down. Then roll it in the paint booth and shoot it. Keep in mind that
some states frown on people painting vehicles without a paint booth.
Check the laws first.