Undercoat: a base color coat applied onto the primer coat.
Primer: a product specifically designed to promote adhesion between a substrata (metal, wood, fiberglass, etc.) and the following paint layers.
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I very seldom spray an undercoat. I do spray a nice thin coat of primer (usually Krylon) on minis. 4 or 5 passes and turning the mini a quarter turn each pass with a final pass across the top. I'll try to do several minis at once to maximize the use from the spray can.
After that is dry, then I do either a base mid-tone coat and work each direction (shadow wash - highlight), or I do a dark-tone coat and work toward the light.