LP-04Seal or size the wall before paperingBrush on a coat of size — wallpaper paste diluted with a bit more water than usual — and leave it to dry overnight. This matters most on bare or freshly plastered brick/block and on new, unskimmed plasterboard, both of which are porous and will suck moisture out of your paper unevenly. On dot-and-dab plasterboard, size lightly and don't soak the wall, as too much water can weaken the adhesive dabs behind the board. On lath and plaster, use a fungicidal size and again keep it light, since old plaster can loosen from the laths if it gets too wet.