Mirror Removed – Ready for Wallpaper? NOT!


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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