Can you spray or roll on wall texture over an already painted wall?
If you want the wall smoother so you can apply wallpaper over it, I would first prime the wall with an oil primer (because it is painted in semi gloss) such as Zinsser’s Coverstain or XIM 400 (provide good ventilation during and after painting). Then you should skim coat the walls with drywall compound to smooth out the texture thus giving you a smoother wall to wallpaper over. Skim coating is the process of applying drywall compound to the entire wall. This is done by applying the compound with a broad knife of 12 ” or larger. You will probably need to skim it twice to get it smooth – allow the drywall compound to dry between coats. Sand the compound smooth. Vacuum off the dust and prime with a new construction drywall primer. After the PVA primer has dried I would re-prime with a wall covering primer such as Zinsser’s Shieldz. You will have a smooth wall ready for wallpaper at this point.
