I have an old home that has previously painted wallpaper in all the bedrooms. All the seams are showing and as you can imagine it looks terrible. My home is lath and plaster so I have no intention of attempting to remove the painted wallpaper. My question is this...if I get the wallpaper liner, apply vertically, paint with primer and then a latex coat-should I sand or spackle the existing seams before applying the liner?