Hi all, In one of our rooms two walls are external and are plastered and painted. Obviously I'll size these before I hang the wallpaper. The other two walls are plasterboard/drywalls. What preparation is needed before I hang the wallpaper? Oil-based primer/sealer? Which one is best (links if possible)? I presume I still size the wall after? I intend to hang lining paper first - does this need to be sized before the wallpaper goes on? Many thanks, Paul.