Spray Primer Basics
Once you've assembled your miniatures and allowed whatever adhesive you've used to set fully, it's time to undercoat your models for your painting "canvas." It's best to make sure that there's no debris on your miniatures before you start applying the spray primer, as the primer will seal in whatever is attached to the model and that might ruin the surface. Sometimes, curls of plastic from trimming your figures or even dust and hair can be resting on your models either from exposure or a slight static charge. Give your models a light brushing with a Tank Brush or give them a good blast of air from your lungs.
In the case of plastic model kits, it's best to wash the plastic with soap and water before undercoating the miniature with spray primer. Plastic sprues are coated with a mold-release agent during the production process. This agent causes a chemical reaction when the primer is introduced, which pits, scars, or even "fuzzes" the plastic figures. In summary: you will cry great buckets of tears when you find that all your hundreds of plastic Imperial Guardsmen that you painstakingly trimmed, assembled, and converted are completely fuzzy and devoid of detail because you didn't clean them first. That is, unless you're into that sort of thing.
Sometimes, you may find that your metal model components are covered with a thick layer of talcum-like powder. If this powder is present, it's always a wise idea to scrub the pieces down with soap, water, and an old toothbrush. The powder, a casting byproduct, may make it extremely difficult for superglue to form a bond during assembly and the spray primer may react strangely to this fine layer of dust.
As an aside, it's advantageous to use spray primer to undercoat a model instead of regular acrylic paint applied by hand. Primer is specially formulated to bond to metals and plastics. Acrylic paint, being water based, has trouble adhering to smooth plastic and metal. To get a proper undercoat by hand, you'll need to apply a few layers. In doing so, you run the risk of obscuring detail. It goes without saying (but we'll say it anyway) that undercoating your models with spray primer is much faster than doing the whole process by hand. Also, subsequent layers of paint flow more smoothly on miniatures that are spray primed as opposed to those done by hand.