I have taken the wall paper off of a wall, to find there are several layers of paint under it which. I have sanded till it's as smooth as I can get it, but there are still patches not even 1 mm deep that look like little indentations. I have bought polyfilla smooth over, but when putting this on, layers of the previous paint are coming off and. The peeled paint is getting stuck to the roller, and then making the rest of the surface have lines across it. Should I sand more and risk damaging the plaster? Any help would be appreciated as I am a total novice. Many thanks