When spraying large body pieces should a primer be set first?

Using primer is more dependent upon other factors, in my opinion. First, is the type of color coat - if it is very different from the plastic color, primer is usually called for. Second, is if the kit contains parts of different materials (or colors) or required lots of filler. You need to get a consistent base for the color coat here. Finally, some paints need a certain color beneath them to look right. For example, the current F1 Ferrari red spray by Tamiya really looks best over a white or pink primer coat. There are other reasons for a primer coat, like protecting the plastic from 'hot' paints...