That is mirror mastic and it is not coming off without ripping the paper off the drywall. You may want to demo and install new drywall.

I would try Citrasolve. I have used it with great success for removing all types of glues and mastics. No harsh smells or chemicals. Use full strength with a scrubby to soften the glue, then a putty knife to scrape it off. Admittedly, I have not used it on drywall, but it works well on other surfaces, even clothing. I purchase it at Whole Foods.

I agree with Matt...however, do be advised that unless your skimcoat is perfect, wallpaper will not look good. If you must use wallpaper (and the self stick stuff is so much more difficult to hang than if you use pasted, imo) do yourself a favor and get a very busy pattern.

Better yet, trial a small area so you don't waste time/money.

"do be advised that unless your skimcoat is perfect, wallpaper will not look good."

Which is why I suggested replacing the wallboard.

What MillWorkman said, the area is not that big it will be much faster and easier to cut that out, put a new piece of sheetrock and a few coats of Spackle.

We do this with back-splashes all the time when replacing them, especially nowadays with quick drying compounds you can do all the coats in a few hours and do wallpaper or paint on the same day if you that good and quick with it... if not and you have a painter who specializes in finish work, it would be a breeze :-)

Good luck

I wouldn't recommend hanging drywall in this instance....unless he/she has done it before, successfully....which I don't know with certainty, but am guessing that they have not due to asking if they should "patch the wall before placing wall paper?"

This was (also) posted in the DIY section of the forum. My assumption is that they are looking to DIY...

Well, try the citrasolve...I hope it works for you. May as well start with the easiest solution and then work your way up if it doesn't work.

Those glued on mirrors are beastly to deal with when it's time for them to go.

Good luck with the wallpaper. I hate it personally and wouldn't buy it again...it just sticks to itself and that makes positioning a nightmare, whereas pasted wallpaper "slides". I do know others here who are more successful than I was, though.

Good luck and much success with your project!

"I really hope Citrasolve will work"

NOTHING will break down mirror mastic other than a prybar or a shovel unfortunately.

I've never used it myself, but I had an aunt who soaked some used delft tiles (that she had pried off somewhere, so they had plenty of "stuff" stuck to the back) in wd40....I know that wouldn't work in this case, but maybe some solvent might help. Personally, I too have simply replaced the drywall (on a backsplash) because I don't have much patience :) But then, I know how to do that and the op doesn't.

Who knows? And fingers crossed for you newproject. Let us know, okay?

I think DIY skimcoat to get a perfect flat surface for wallpaper would be tougher than changing a sheet or two of drywall but maybe that is just me.

I would NOT use Citrasolve. You will end up with a gooey, oily mess if it works. Melted black mastic will get every where. And then you'll have an oily residue that has soaked into the paper and gypsum in the drywall.

I would scrape it off and deal with repairing the drywall.

I am saving money to remodel this space but didn't want to live with this meanwhile, so I tried Citrasolve. It was not prefect but worked! I removed most of glues and put some sticky wallpaper on it.

If you didn't seal w/ a couple of coats of shellac, the black stuff & oil may bleed thru the wallpaper. If you don't have leftover paper, I would pick up a spare roll to have on hand (while the pattern is still available) just in case you have to do a do-over.