KNOW HOW

A Primer on Drywall Repair

By Jeanne Huber
Special to The Washington Post
Thursday, April 20, 2006

Q My guest bathroom had wallpaper. A friend took it off for me. But lots of the drywall paper came off with the wallpaper. What should I do now?

ANow you know why decorating experts always advise painting drywall with a suitable primer before the wallpaper goes up.

With a good primer underneath, wallpaper is easier to install, because the adhesive stays slightly slippery, so you can nudge sheets into alignment. And when it's time to redecorate, the primer helps the wallpaper come off relatively easily, leaving the wall intact. Without the primer, the paper face of the drywall absorbs a lot of the adhesive, which makes the wallpaper more difficult to hang and remove. What happened to you is all too common: Because the adhesive soaked into the paper face of the drywall, removing the wallpaper also strips off a lot of the drywall covering. You're probably left looking at ragged, somewhat fuzzy brown paper, which manufacturers use underneath the paper face to help hold together the gypsum core of drywall.

But there's no use whining now over shortcuts taken years ago. Your next step should be to paint the wall with a primer that seals especially well. Standard water-based primers won't work because they don't block moisture well enough. The surface needs to be sealed or the remaining paper will bubble up when you attempt to smooth over it with drywall mud or even just paint.

For years, experts recommended sealing damaged drywall with pigmented shellac, which has an alcohol base, or with an oil-based primer, which contains petroleum solvents. But both of these generate fumes you probably don't want to breathe. Today, there is at least one water-based alternative that's specifically designed for this purpose: Gardz, manufactured by Zinsser Co. Best known as a shellac supplier, Zinsser specializes in primers that solve specific problems.

Like most water-based primers, Gardz is based on acrylic resins, but it uses a specific type with unusual properties. The same acrylic is used in products that stabilize crumbling concrete and in some specialized joint compounds, according to Tim O'Reilly, who oversees primer products for Zinsser. In the can, the primer is clear and as thin as water. "On the wall, it soaks in," O'Reilly says. "It turns rock-hard and gives you a new surface that can be mudded over or primed and painted."

"Mudding over" is a process also known as skim-coating. It's the best way to even out the surface on your walls if they aren't smooth enough to simply repaint. You apply a thin layer of drywall mud, allow it to dry, then sand the surface smooth. It's not particularly hard, but it is time-consuming and messy.

Use standard drywall mud, which is also known as joint compound, not spackling paste, lightweight joint compound or quick-set joint compound. Before you begin, spread dropcloths over the floor and use painter's tape (often blue) to mask off trim or other surfaces you want to protect. Stir the mud to a smooth consistency and ladle some of it into a small container that's easy to carry. Apply the mud with a wide drywall knife. Some people prefer a five-inch-wide blade, while others like tools that are 12 inches across or wider.

Work from the top of the wall down. Smooth on the mud vertically, not in random curves. Aim for an even layer about 1/16 -inch thick, but don't worry if you leave slight ridges. As you work, long crevices may start to appear behind your knife at some point. These are caused by dried bits of drywall mud. Pick them out, if there are just one or two lumps; if there are many, scrape out what's left of the mud in the tray and reload it with a fresh supply.

While the mud dries, which could take four hours or overnight, depending on the weather, tape up plastic sheeting over doorways or the section of the room where you will be working. The next step generates dust, and you want to keep it out of the rest of your house.

When the surface is dry, scrape across the wall with the drywall knife to remove the ridges, then sand the surface smooth. Misting the wall with water first helps keep down dust, or you can rent a drywall sander hooked to a vacuum that's designed to deal with the fine dust made by drywall mud. You can also use standard sandpaper (120 grit, followed by 150 grit), but be sure to wear a disposable respirator and maybe a hat or a shower cap. When you're done, brush clean the sanding debris from the wall and your clothes, and let the dust settle. Then inspect the surface carefully by shining a bright light at an angle over the surface. If you see holes, skim over the wall again with drywall mud, but this time don't add to the thickness, just fill indentations. Let those dry and spot-sand, as necessary.

With a smooth surface restored, you should then prime the walls again. You can use leftover Gardz for this, but it costs more than other primers, so if you need more, switch to a standard primer. If you plan to put up new wallpaper, use a primer-sealer or a wallpaper primer, which is similar but contains a tiny amount of wax to help with positioning of the paper. If you plan to paint, use a PVA primer, which contains polyvinyl acetate, the same resin used in white glue.

And then, at last, you can put up new wallpaper or roll on the paint of your choice.


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