After steaming & scraping off three layers of wallpaper on this wall, here's what I found: plaster unevenly meeting the wallboard where previous owners apparently covered a doorway. I'm not sure what to do. I found it nearly impossible to remover paper from the plaster without ruing the plaster!
Rick J, Wilmington, DE
What a mess! The bad patch job by the previous owner has left you with a real predicament. You have two options:
1. Spackle - Build-up the difference between thicknesses using several layers of spackle. I would start with a 4 inch spackle knife, then switch to a 6 or 8 inch blade, and then a 12 inch. Each layer should be put on as thin as possible. Sand between coats. The idea is that you need to taper out the spackle from the botched patch to the finished wall.
2. Rebuild the wall - For this you will need to remove the trim on the doorway,remove the bad patch, frame out the wall (or trim it) so that the new patch will be flush with the old wall. Consider using 3/8" sheetrock if 1/2" is too thick. I'd probably take some of the old wall with you when you cut out the bad section. You will need to decide whether you want a horizontal seam or a vertical one, and then cut accordingly.
In either case, the wall isn't going to beas smooth as you'd like it to be so I'd opt for a textured paint finish, which will hide many sins - including those of the former repair person!