It is sometimes assumed that wallcovering will hide discrepancies and flaws in the wall surface. This is not always the case. Flaws in the wall surface like nail holes, small bumps and grit, even texture caused by brushing and rolling paint, can sometimes show through a layer of wallcovering. Wide open patterns, light backgrounds and shiny wallcoverings are more likely to show wall surface flaws.
I always try to remove the wallpaper because this is the preferred method. Prominent seams and irregularities in the previous layer can show through the new layer. Bubbling of the first layer can also be a problem.
Priming provides a sound surface that wallcovering will adhere to. Priming will aid immeasurably when it comes time to remove the wallcovering for redecoration. If you are sure that your wall surface is sound and adhesive friendly, priming may not be necessary.
For years, oil base primer was the way to go, but with recent passage of clean air laws, manufacturers have been forced to change their formulations. Complying with these VOC laws has changed the properties of the finish surface, so that it is no longer a reliable adhesion promoting surface.
Removal of wallcovering can range from a cake walk to a frustrating, exasperating experience. Type of wallcovering, adhesive and prior wall preparation will be the determining factors. You will most appreciate proper wall preparation at removal time.
Double cutting is a method used by professional paperhangers to make a seam on the wall rather than trimming on the table. It is often superior to butting the factory edges, and with some wallcoverings, commercial vinyls, for example, it is the prescribed method.
Sizing originally referred to a glue like material that was manufactured from animal hooves, hides and bones. It was used to coat a wall surface prior to wallpaper installation to even out porosity and provide a better level of adhesion. Improvements in paint and adhesive chemistry have made size redundant. Nowadays, the word "size" is often incorrectly used to describe most anything you put on a wall, prior to wallcovering.
Liner may be a plain paper (known in the trade as "blankstock") that is applied to a wall surface prior to the decorative layer of wallcovering. It also may be a "bridging type" that seeks to provide a new surface when the existing surface has voids. The bridging type is more common, often pure white in color and is often composed of at least 50% synthetic ingredients such as rayon and polyester. Both types act as a buffer between the wall and the second layer of wallpaper.
