I have a house from 1944. Redoing the kitchen and pulled off formica backsplash to reveal thick dark brown adhesive. Used a power scraper to get the heavy thick stuff off. Now I'm left with the rest. I very uneven. It's not water soluable. It doesn't react to a heat gun, and today I was attempting to sand the hell out of it. No dice. Help!!
What are you planning on having for a final finish?
That stuff is nasty, I've seen grown men weep out of frustration trying to remove it.
It's easier to take out the plaster board down to the studs and re-blend in new blue board and plaster.
(I've seen contractors do just this)
You'll have to chisel, then resurface if you want to paint.
The easiest way out is to cover it over with new tile.