Still amazes me how many hobbyists forgo primer and will instead either paint directly on the surface or will use a matte spray paint as their base coat. I’m no expert, I can’t tell you the exact science of what makes a primer a primer, but I can tell you that it does make a difference. The short of it is that a primer adheres better to your models than normal paint and it also gives you a better surface to paint on. Primers are created to stick to the surface you’re spraying and to receive paint. By not using a primer you are only hurting yourself. Your paint is less likely to stick to the model and in turn more likely to come off either through handling or by dropping the model. Most of us seal our models and even sealed the models that were primed are less likely to chip and take damage than those which were not primed. In short, all layers of paint are adhered well to the model’s surface. If your first layer of paint is not well adhered, you didn’t use a primer, than impact could cause much larger chipping from being dropped.
So what primer should you use? I prime all my stuff in black and so my experience has been pretty much in that area alone. Black primers, for me, are hard to come by. The first primer I used that I liked was the Armoury black primer. It’s my understanding that they went out of business years ago but you can still find it at some gaming stores. It’s a bit pricey but it’s a larger can and it goes on well. I know some use Krylon’s primers and I’ve seen mixed reviews about their black primer. I can’t find it in black to save my life so I can’t say for sure. However, I use Krylon for sealing and I’ve always had great luck with their products as a whole. I’ve used a black primer from Brite Touch, sold in automotive stores, and it’s horrible. No matter how much you shake the stuff it just goes on like water and the pigment is weak. What I’m currently using is by Dupli-Color, another automotive primer, and I’ve been having great success with it. They have a bunch of specialty primers so be careful what you get. I use their sandable primer, basically their run of the mill primer. Don’t get their filler primer or anything like that, bad things will happen. Dupli-color also has their primers in white, that lovely rust red color, and gray.