Drywall, Primers and Coatings. Experts offer their thoughts on using primers, skim-coats or surfacers to get the best possible finish.
by John Strieder
in a sandwich of thick paper. But it gets a lot of love.
Code enforcers like drywall because of gypsum’s ability to retain water. It steams when exposed to flame, so it’s a solid fire retardant.
Builders like it because hanging gypsum board takes only a fraction of the time needed to plaster a wall. Just nail it up, set joint compound and tape in the valley created by the slabs’ tapered edges, and slather in more joint compound to fill the valley. Presto! A finished wall. Well ... almost.
Even after sanding or sponging, a gypsum-board wall is not ready for paint. The soft crown of joint compound looks nothing like paper, and doesn’t accept paint the same way either. How does a contractor create a smooth, attractive surface from such muddy beginnings?
The biggest problem is porosity. Drywall paper and joint compound absorb moisture at different rates, and the paper accepts paint unevenly from one square foot to the next. So the first coat of paint will likely result in a very patchy finish, with shiners and dead spots. The glossier the paint, the bigger the problem.
A coat of semigloss on drywall can turn satin or flat, says Steve Revnew, architectural coatings marketing director at Sherwin-Williams Inc. That’s why, like most experts, Revnew says a preliminary coat of primer is essential when painting drywall with anything other than flat paint. “If you do not use primer, you’re not going to get full gloss and sheen development,” he says.
Jeff Broker, who markets interior finishing systems for U.S. Gypsum Co., agrees. “To ensure a better finish, it’s always recommended to prime it,” he says. “A good heavy-bodied primer will equalize those surfaces.”
It’s true, multiple coats of high-end paints will also smother the flashing problem. “Two to three coats of eggshell may be enough for holdout,” says Ron Boyajian, product marketing manager for California Paints. “More often than not another finish coat gets the job done.”
But a primer will promote adhesion, a good reason to use one even with a flat, Revnew says. “Could you get away with not using it? Yes. But standard flats will not always have great adhesion to drywall.”
Primers create a protective barrier against stains, odors, and in some cases moisture vapor. They even protect a finish from the drywall itself. The paper in drywall is often recycled paper, saturated with waxes and inks that bleed onto the finish coat, Boyajian says. “It’ll look like a little shadow.”
As for application, primers spray just like paint, with only the same difficulties posed by spraying paint. “You’re subject to all the pitfalls in primer that you would find in a finish coat,” he says.
Gypsum board manufacturers promote another method of sealing drywall: skim-coating. A thin layer of joint compound, spread with a trowel across the entire surface of the drywall, solves the porosity problem. It plasters a smooth textured finish over paper and joint and masks other imperfections in the drywall.
When it comes to drywall, smoothness is a big deal. So big, in fact, that industry trade groups have devised a chart that describes five grades of finish and how each is achieved. Thanks to the chart, contractors, architects and building owners know they are speaking the same language when they spec drywall. (See sidebar.)
A skim-coat of joint compound or specially made skim-coat material is mandated at level 5, the top level of drywall finishes. But skim-coating is costly, says Lee Jones, spokesman for the Gypsum Association, a manufacturers’ trade group. “If it’s not necessary, it’s truly expensive, an unnecessary expense.”
So its no surprise that manufacturers are busily developing spray-on, high-build products that offer the benefits of both primer and skim-coat in one surfacer.