Every great journey begins with one small step. And when it comes to hanging wallpaper, the first step is one of the most important. Wallpaper can’t cling to greasy, dirty walls, old wallpaper or paint. That’s why we can’t emphasize enough how critical it is to have your walls properly prepared.
Newly plastered walls and ceilings need to be sized (sealed). New plaster is very porous. If you apply any kind of ordinary paste directly onto new plaster, moisture will immediately be sucked out of that paste. In simple terms this leaves the paste going dry on the surface only and because the moisture has been sucked out so quickly, it has no "roots" or "key". The paste may soon start to lose its grip.
To seal a surface before you hang wall paper the wall also needs to be totally dry. The reason for not sizing (sealing) before the wall is completely dry is that it will form an air tight skin over the wall. The moisture from the new plaster is then trapped behind this skin and cannot evaporate off. The damp then either retreats back into the wall where it develops mould growth or reacts with the salts in the wall.
Sealing a surface for papering can be done using a proprietary product called size. This is a gelatinous solution which is mixed with water and applied straight onto the wall and allowed to dry. It can be papered over while still wet but, with the addition of wallpapering paste it sometimes makes the wall too slippery to hang paper properly. A dilution of the wallpaper paste you are going to use can also be used as a form of size to seal the wall. Most pastes will have a sizing solution on the packet but a rough guide is to use 25% more water in a sizing solution.
If you intend to apply a vinyl paper to the surface, it is best to prepare the walls by sizing with a dilution of a paste with fungicide in it. Vinyl papers are air proof and as a result any dampness at all that is trapped behind the paper will not be allowed to evaporate out. This could turn to mould. Apply the size or diluted paste with a large emulsion brush.