Originally Posted by syogod
Is it necessary to paint over a primer? I have primed a previously painted ceiling and am ok with the color of the primer.
As Surf said, primers are formulated to behave differently than finish coats - though primers are less
, not more, porous than finish paints (especially when you're talking about ceiling paint as your finish)...but he was right when he said primers will collect more dirt. And Jason said ceiling don't tend to collect dirt like walls do, and that's true if all you're talking about is handprints and the like.
The reason primers will collect more dirt than wall paints is due to type of resin and specific additives that primers possess that finish products may not. You can have both an acrylic primer and an acrylic finish yet they could be completely different resins. Some acrylic resins dry hard and are somewhat resistant to burnishing and have a degree of stain resistance - these types of acrylics would be more commonly used in finish paints since both of those characteristics would be desirable in a finish...
Other acrylics may dry to a softer, "gummier" film that is not as stain or burnish resistant, and more apt to soften (even more) with heat and humidity as opposed to another type acrylic. These, then, would typically be used for primers since the softer, slower dry finish is necessary in primers to bind any remnant sanding / drywall dust while still providing adhesion to the surface.
So...if you leave a primer as a finish, and because the resins are probably those that I've described, the dirt that the film collects is mostly air-borne particles that settle on the soft finish, become embedded into the film (as it softens and re-hardens during periods of high humidity), then could possibly create issues when it comes time to repaint.
I'm generalizing. I have no idea what type of resin is used in whatever primer you're using, but if this particular primer is designated as a multi-purpose primer (wood, drywall, plaster, plastic, small children, etc.), then I'm probably right and wouldn't want to take the chance on it. Good luck.