Mirrors are commonly attached to walls with mastic – a sort of tar adhesive. When installing this new mirror, the workmen got some of the tar smeared on the wall below.
This is bad. Tar, like many other substances, will bleed through paint and wallpaper.
So this stain will need to be sealed over with a stain blocker. Or, better, cut out of the wall entirely, and then the damaged wall will have to be repaired. Learn more on this by doing a Search in the upper right corner.
A mirror had been glued to this wall with mastic adhesive (a tar-like substance). When the mirror was pulled off the wall, the adhesive pulled some of the drywall along with it, and in other places it left some of the tar on the wall. Then someone skimmed over the surface with joint compound.
The wet joint compound caused the torn areas of the drywall to absorb moisture and ripple, and the tar worked its way through the joint compound.
Both torn drywall and tar are problems under wallpaper. The ripples from the torn drywall will show under the new wallpaper. And moisture from the wallpaper paste is likely to make the bubbles larger. The black mastic (tar) will bleed through the wallpaper, creating black spots.
If I had been there when they removed the mirror, I would have taken a utility knife and cut the globs of mastic completely out of the wall. Removing it is preferable to trying to cover it up. Yes, this would have torn the drywall, opening it up to wrinkling when it gets wet with primer or paste.
But the penetrating sealer “Gardz” is designed to fix torn drywall. It dries hard and impermeable, so moisture cannot get through. No worries about bubbles or wrinkles! The cut areas could then be skim-floated over and then sanded smooth.
But since I didn’t get to prep from the beginning, I inherited this wall in the top photo, with torn, wrinkly areas, and with tar bleeding through the joint compound.
To prevent additional bubbling, I coated the wall with Gardz. Once that was dry, wanting to both smooth the wall and create an additional barrier to contain the mastic stains, I skim-floated the entire wall, let dry, sanded smooth, and sealed again with Gardz.
Gardz doesn’t protect against stains, though. So, to keep the mastic from bleeding through, I coated the wall with KILZ Original oil-based stain killer and blocker. This worked better having the joint compound under it, because when I’ve put KILZ directly on mastic adhesive, the two petroleum-based products simply melded into one another, and left us with the very real potential for bleeding through wallpaper (or paint, BTW).
So the KILZ should have effectively blocked any stains from the mastic. But the new problem is that wallpaper paste will not stick to modern, EPA-approved, oil-based products. Plus, I was worried that a little of the black tar might still find a way through.
So I skim-floated the wall again, creating yet another layer that would bury those tar stains. After that was sanded smooth and wiped free of dust, I applied another heavy coat of Gardz.
All this took a long time, but it’s good assurance that bubbles will not be seen under the new wallpaper, and that no black spots will grow on its surface.
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This powder room in the Medical Center area of Houston had a mirror glued to the wall. The homeowners want a different mirror, so had the original one removed. The mirror was attached to the wall with a tar-like substance called mastic. The glass guys use suction cups to clamp onto the mirror, then they pull it away from the wall. Some globs of mastic will be left on the wall, and this is a problem, because these tar-like substance will bleed through the new wallpaper.
So, to prevent any mastic / tar residue from bleeding through the wallpaper, the handyman took a Stanley knife / box cutter and cut out the top paper layer of drywall that had any mastic on it.
To smooth over the uneven edges, the handyman skim-floated the area with joint compound. The moisture in the joint compound caused the exposed paper inside the drywall to swell.
So what you are seeing in the top photo is a wrinkled section of drywall caused by moisture. This will show under wallpaper.
So I took a Stanley knife and cut out the handyman’s patch, to remove the wrinkled drywall paper layer. I sealed it with a penetrating primer called Gardz, which soaks in, binds surfaces together, and dries hard.
Once that was dry, I skim-floated over it (and the entire area), to get a smooth finish. Once my skim-float layer was dry, I sanded it smooth, and then primed again with Gardz.
The Gardz did its job, and did not allow moisture to penetrate into the exposed drywall, so no more wrinkles developed. See the second photo. Now the wall is nicely prepared and ready for wallpaper.
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Mastic is petroleum-based, and it, like other similar substances such as grease, oil, and crayon, as well as other compounds like blood, rust, water, tobacco tar, and others, will work their way from behind the wallpaper up through it and then onto the surface, causing an unsightly stain.
KILZ Original oil-based primer and stain blocker is a superb product for sealing these substances. However, I feel more confident if the suspect material is removed entirely.
The best way to do this is to take a Stanley knife (utility knife / box cutter) and cut around the stain and into the wall. Then you can use a stiff 3″ putty knife to peel up the top layer of drywall, taking the staining material with it.
This leaves a patch of Sheetrock without its protective top layer. See Photo 3. These layers of torn Sheetrock will absorb moisture from anything you put on top (paint, primer, joint compound, etc.), and will swell, creating ugly bubbles that will mar the finished job.
So I brushed on Gardz, a penetrating sealer / primer by Zinsser. This is cool stuff, because it soaks into the surface and then dries hard, binding everything together.
In Photo 4, I have skim-floated over the areas where I have cut out the mastic. To skim-float, I trowel on a smoothing material called joint compound. Once that is dry, I will go back and sand it smooth, creating a perfectly smooth surface ready to accept the new wallpaper.
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