Wallpaper sizing is a form of sealant that you apply to the wall before wallpapering. The sizing paste makes the job of applying wallpaper easier for the do-it-yourself homeowner or professional.
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Wallpaperers treat or coat the walls with a watered down or diluted wallpaper wheat paste prior to a papering project. Older wallpaper hangers use this practice and the term sizing, which is, in effect, a form of priming.
In the past sizing prevented the moisture from wallpaper paste seeping through the painted wall too quickly and causing difficulty with the smoothing and adjustment process. Today sizing is necessary for "true wallpaper coverings", according to the Wallpaper Installer website. An installer adds sizing to the adhesive when working with a cured, newly plastered wall.
Sizing or wall primer/sealer is hand-painted or rolled onto the wall being prepared for wallpaper. A series of sizing or primer products are available for the different types of wall surfaces.