There are two types of primer: 1) Brush on, and 2) spray.
First know that the reason for priming is so that the paint being used with adhere to the character. Metal is to slippery for most paints, but coated lightly with primer it sticks very well. So what type of primer should be used? Most hobby/craft stores that carry primer mark up the price. I couldn't find any store that carried brush on primer, but they carried spray primer; it cost $12.00 (USD). I then drove to a local general type big-box store where there was spray auto primer for $3.77 (USD), which can held 25% more paint then the hobby store brand. Problem, which type of primer paint to buy for there was gloss and matte, light color and dark color? As for color I went with the lighter color, but many use black as the base color first, so dark might work well (but light wasn't a gamble). Choice two, glossy or matte? Matte is better for it is more natural looking and glossy might cause a problem of paint sticking (of this I don't know).
After purchasing primer the next step is attaching the miniature to something that will hold the model so that it can be painted. A stick, dowel, spray paint lid, whatever is comfortable for you, and you can find. Tack the figure to the holder with some glue (I used Elmer's all purpose glue). This holder will be used not only for the primer painting, but for the general painting as well. For a holder I used a very short pencil that had no eraser.
|Tacked to holder before priming|
NOTICE - Don't hold the can to close to the target! Spray from a distance of 12-18 inches, and use short bursts. The spray takes very little to cover a 28mm figure. A couple bursts on each side. Make sure the paint doesn't pool or run on the figure. If it does suggest using a cotton swab to absorb the paint (this goes for the entire painting process). Also look to make sure the paint doesn't bubble. The spray primer will first look like it is glossy, but it will dry matte.
After priming one can see how detailed the carving is. To much primer or paint will cover this detail, and one doesn't want that!
Let the primer dry. Some primers will dry very quickly, but I suggest 24 hours to be sure.