Size is a thinner kind of wallpaper paste, which can be bought from all good DIY stores. Just like wallpaper paste size is inexpensive, but what a difference it makes in the finished quality of the look at the end of your project. Simply follow the manufacturers instructions on how to mix and apply the size.
Applying size to a wall or ceiling prior to papering prevents wallpaper shrinkage. This avoids the gaps that can appear between the lengths that you have hung once the paste has dried.
On newly plastered walls the problem increases as freshly plastered walls and ceilings will suck paste out of the paper. This can be prevented if the plastered surfaces are sealed before doing anything with them.
The surfaces can be sealed using a solution made up of white matt paint and water.
Your newly plastered walls or ceilings are now prepared and ready for wallpaper/decorating.
On previously papered or painted surfaces, an application of size is required.
Once Dry: