I've grown quite accostomed to Armory Primer, available in white, grey, or black. It's mainly used by miniature painters, especially metal ones. I just like to have a completely uniform surface before shooting the actual model. Black primer is great for models that need a darker base color, such as most tanks and a few planes, while works well on brighter colors.
In answer to your question, mannyhern, I've used laquer primers and painted over them with enamels with no problem. Just don't reverse it, or the results will be disastrous! As to whether priming cars over A/C is more important, it's totally in the discretion of the modeler.
demono69