On a big painting project, laying down a coat of primer can feel exhaustingly unnecessary. But primer will likely be the difference between a 5-year exterior paint job and a 25-year one. Primer serves two main functions: It seals in any remaining moisture (which prevents mildew) and provides a smooth, clean surface so the topcoat of paint will stick like glue. If you are lucky enough to start with a perfectly smooth and clean painted surface, priming isn't necessary, but few of us are so lucky [source: Paint Quality Institute].
Before you apply primer, you need to mask windows, gutters and any other unpainted surfaces with blue painter's tape. Then, spread the primer on with a sprayer or roller and follow up with a brush to work it into hard-to-reach corners [source: Lernley]. We'll talk more later about the pros and cons of using brushes, rollers and sprayers.
What about oil-based versus latex primer? Professional painters prefer latex primers and paints because they are water-based and clean up easily [source: Dawson]. But some choose oil-based primers that deeply penetrate wood surfaces [source: Lernley].
Do you need two coats of primer? Again, the smoother the painting surface, the better-looking the final paint job, but you need to consider how much time and money you want to invest in this project. Primer is cheaper than paint, but it's not free. A 5-gallon (19-liter) bucket of exterior primer starts at $60 to $70 and goes way up from there.