Priming is the first treatment applied to a bare, untreated or highly absorbent surface before the actual paint layers. The primer ensures good adhesion between the surface and the following coats, and evens out the absorbency of the surface so the topcoat dries to a uniform finish. A stain-blocking primer also prevents damp patches, stains or knots in wood from bleeding through the new paint.
Which primer for which surface?
The right primer depends on the surface, the condition of the existing paintwork and the paint system being used. A new or freshly plastered wall absorbs paint quickly and unevenly, which leads to streaking, so a wall primer is applied first. For wood, there are primers for interior and exterior use; exterior primer is resistant to moisture and temperature changes. For metal, a rust-inhibiting or anti-corrosion primer is used. For plastic and MDF, adhesion primers bond well to these smooth or composite materials.
When is priming not necessary?
If the existing paint layer is still intact and adheres well, a new primer coat is usually not needed. Cleaning, degreasing and light sanding is then often sufficient. Bare patches are, however, spot-primed first.
Why you should not skip the primer
Without a primer, there is a greater chance that the topcoat dries unevenly, adheres poorly and eventually starts to flake. Have a professional assess whether priming is needed, and which primer, before you continue painting.