Drywall Primer Video
When finishing drywall, the drywall paper is a different texture than the drywall mud or joint compound. Painting directly over these two surfaces will not provide the results you want. The drywall compound absorbs paint at a different rate than the drywall itself. In addition, the drywall surface is not as smooth as the joint compound.
There are paint primers designed especially for drywall. These primers have a resin that seals the surface, so the next coat of paint will not absorb at different rates. The prime has a heavy pigment to help cover the little imperfections in the drywall surface.
The primer paint can be tinted in several shades of gray. The tint is matched to the finish paint color. In many cases, only one finish coat of paint will be required. What a timesaver!