Good Morning, I am new so I hope I have put this post in the right place. I had some border paper removed from one of my rooms. The guy I hired sprayed this liquid all over it and it came off easily. He then spackled over the area that had the border paper on it. I noted long streaks of liquid running down the walls. I asked if he was going to clean them up. He said it didn't have to be cleaned off and I could just paint over them. So I did. NOPE. There are now streaks on my walls and the liquid also ripped off the paint on my ceiling and on the corners between the wall and the ceiling. This is what I did after realizing just paint wasn't working and trying to clean off the now dried liquid wasn't working: put on 2 coats of KILLZ with proper drying time, then painted with Ben Moore 2 coats with proper drying time. I also used the blue tape properly on the corners, proof of which to me is that I have done 4 rooms in my home and on all 4 rooms the corners are near perfect with straight edges and no touch up needed. Any suggestions on what to do now? I am thinking that I am going to have to sand down all the walls to get this stuff off before I can re-do all of that work but I don't want to do that if there is a simpler solution. Thanks!

I am applying tongue and groove siding (vertical application) to the interior walls of a home. I don't have any problem with the actual application, my question is "What is the correct and most effective way to prepare the walls before the final application. Like do I need to put up backer board (as used in Tile work) to make sure that my walls are uniformly plumb before I start. I don't claim to know all of the fancy construction lingo so please bear with me. Thanks JOHN