John,
I can share my experiences with you, but it may be an apples to oranges comparison. I’m restoring a 1850 Gothic revival. Every room was wallpapered, including the ceiling. Keep in mind this house has no sheet rock, but lathe and plaster and plaster over brick. You can be assured this was the old brush on paste. Much of the wallpaper was painted in later years.
Here is what finally worked for me. I’m not suggesting you do this, it’s just what worked for me. I scored the wall with a utility knife in a small diamond fashion. The turtle did not penetrate all the layers of the wallpaper and paint, so I had to use a utility knife. I mixed water with fabric softener, just as you stated. But I mixed it extremely hot. I sprayed it on, let is set 15 minutes, sprayed it again. I did this three, sometimes four times, waiting 15 minutes between each coating.The very old paper I was dealing with was thick, painted and easier to penetrate when the solution was really hot. The small diamond shaped perforation that I cut into the wall paper allowed more of the water mixture to penetrate down into the adhesive, since some of the wallpaper had been painted. (The paint kept water from penetrating to the first base adhesive level) I then used a three inch flat blade, red devil wall paper knife to remove the gooey wallpaper. I used 400 grit wet and dry sand paper glued to a flat board to maintain a really sharp edge on the tool. It worked for me…...But! What worked for me, may not for you. There are some obvious concerns for your walls. Since you are dealing with plaster board, you must be concerned with how wet you get it, or more accurately how wet you should not get it and you will get some gouges in the wall board from the razor sharp knife if you are not careful. Those scratches and digs in my plaster were really pretty easy to fix, in plaster board and depending upon your experience level to repair, it may be a burden. Additionally if you apply too much water the paper face of the wallboard will absorb the water causing more damage. So like I said, my experience may be an apple to oranges comparison and may not be suitable for your situation.
I tried some of the commercially available solution that you spray the wall with, but found that hot water and fabric softener worked the best for me.
