It's always wise to prime before painting new interior drywall.
Drywall may be the most commonly used surface found in modern homes, yet it presents some unique challenges for house painters and for homeowners engaged in do-it-yourself painting projects. The experts agree that for best results, new drywall should be primed before painting. There is an ample assortment of interior primers designed for that purpose, but there may also be some surprising options from outdoors.
How PVA Primers Work
One important reason why home builders routinely apply a PVA primer over new drywall is that it's among the cheapest available base coats. But PVA primers -- an acronym that describes its polyvinyl acetate resin -- work extremely well at sealing new drywall surfaces and hiding any routine color variations in the new wall. They also bond any "furring" or small surface flaws back to the surface of the wallboard. That makes PVA primers a fine base coat for the low-grade flat white latex paint that builders normally apply to residential interior walls.
How Upscale Interior Primers Work
White-pigmented shellac primers do an excellent job of hiding stains common to indoor rooms such as surface dirt, greasy hand prints and cigarette smoke. Like alkyd primers, they are also good at blocking odors and grease from migrating through to the new topcoat. Interior acrylic latex primers are available for a variety of special purposes -- they can stop old paint from peeling, block many types of stains and provide additional hide when used over deep-colored painted surfaces. All of these types of interior primers are available as low-odor or reduced odor coatings.
How Exterior Primers Work
Exterior primers are primarily formulated for use on wood siding, although there are varieties designed for use on fiber cement, vinyl and aluminum siding and other special applications. While interior primers are intended to lightly penetrate and then form a soft film over gypsum drywall, an exterior primer is meant to penetrate and bind to a surface with altogether different porosity. Exterior primers form a somewhat harder film than interior primers because they may be required to withstand a few days of outdoor exposure before the topcoat is applied.
How Exterior Primers Perform Indoors
Nearly any general-purpose exterior primer is suitable for use on newly installed interior drywall. There may, however, be a few performance trade-offs. Few of the standard exterior primers can block odors as effectively as an interior white-pigmented shellac primer. Even low-VOC latex or acrylic-latex primers will be smellier than a low-odor interior primer. Exterior alkyd primers will release a much higher level of solvents into the air, which may be harmful when inhaled by children and chemically sensitive adults. Of course, the smell won't matter if windows can stay open until the primer has thoroughly dried.
Comparing Costs
Interior PVA primers are less costly than exterior primers and they provide satisfactory performance on interior drywall. In return, exterior primers offer no performance advantages over interior primers. When indoor walls require a special problem-solving primer, it will usually be less expensive to use an interior primer and topcoat system than an exterior combination. On the other hand, if you have a few cans of exterior primer collecting dust in your basement and you need to paint an interior wall, you should definitely put that exterior primer to work rather than spending money on an interior primer.
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