All About Priming
Primers have many benefits - they improve paint adhesion, help seal porous surfaces, resist moisture, and extend the overall life and durability of the topcoat. While priming never hurts - it's not always necessary in most repainting projects. However, primers should be used when painting over new plaster, new or exposed wood or drywall, bare metal surfaces, or spackling or other touch up work done during preparation.
There are two types of primers - interior and exterior. And both come in latex and alkyd, or oil based, formulas. Interior latex PVA primers should be used on drywall, patched areas, and previously painted surfaces. Interior ALKYD primers should be used on properly cured plaster, wood, and chalky surfaces. EXTERIOR alkyd primers can be used on most exterior surfaces. However, exterior LATEX primers are recommended on non-bleeding wood, exterior plywood, or previously painted surfaces in good condition. If you have questions about which to recommend, simply refer to the primer label. It will list the types of surfaces recommended for that particular kind of primer.