Buy the thickest lining paper available and when you hang it ( horizontaly as opposed to verticaly ) mix up a thick bucket of wallpaper paste and paste the wall prior to pasting the paper and hanging it. This should ensure the paper sticks well and , providing the remaining paint does not resemble a relief map of the himilayas , will even out the wall surface well enough to give a flat looking surface when painted. do not bother buyinf fancy sealers co sanding the walls cos most of the time it is not necessary. Just get some decent wallpaper paste from a decorators merchant and not somewhere like homebase or B and Q.