Primer
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What kind of paint do you have? What do want to 'prime'? Why? Primers perform specific tasks such as bonding to metal, non-porous surfaces, sealing wood fiber, prevent stain bleeding, bonding to hard-to-stick surfaces, and providing a uniform surface so the sheen of the paint is consistent. I have personally used Moore's Aquavelvet on bare drywall with no problems. Para makes a self-priming acrylic sutiable for the same job. However, primer is generally less expensive than paint, gives more bite to the topcoat and masks out color and stains far better than a run of the mill paint. Most times it will work but a paint company would not stand by their topcoat if a problem should arise by taking the shortcut. Here's a link to answer some other questions.
http://www.o-geepaint.com/FAQ/faqu.htm
http://www.o-geepaint.com/FAQ/faqu.htm