Walls should be clean before starting. Greasy walls such as those in a kitchen, can be washed with mixture of water and ammonia (equal parts). Walls should be sanded smooth. Spot patch nail-holes, dents, etc using water-based spackling and re-sand. Do not use oil-based spackling products as they can bleed through the papers. If the wall has a gloss finish, it will require sanding to prepare the surface for adhesion.&

Wall stains caused by moisture, grease, crayon, etc must be corrected before applying wallpaper. Stain-killing primer/sealers should be used to spot-treat these areas. (Moisture problems must be corrected before you install your paper to prevent adhesion or mildew problems)

1. It helps ensure that your wallpaper products will adhere to the wall
2. It makes future removal of the products much easier & protects your wall surface

As wallpaper dries, normally over a 24 to 48 hour period, there is a significant amount of pull as the paper contracts. If the wallpaper has not formed a good bond, it will pull away from the wall. Additionally, if the wall has not been prepared properly when the paper is later removed, it may damage the covering of the underlying drywall which can be very expensive to correct

• Wallpaper Primer/Sealers can provide the best insurance on a good installation. These products are available in several mixtures to address specific needs. A colored (pigmented) acrylic primer/sealer is the most common because it can be used on all surfaces. It's water based, easy to clean and the coloring helps prevent any discolorations from showing through the paper. These products protect the underlying drywall as well as provide a good surface for adhesion.
• Stain Killing Primer/Sealers should be used for walls with problematic stains such as lipstick, grease, crayon, etc. They prevent these types of stains from bleeding through the wallpaper. This product would be used to spot-treat these areas. Once done, an acrylic wallpaper primer/sealer should be used to treat the walls before installation.
• Sizing is generally diluted solutions of paste made from the same paste as found on prepasted wallpaper.  Its use is not to prepare the wall, but to provide added adhesion. It should not be confused with a primer/sealer .

• New Drywall must first be sealed before you install your wallcovering products to protect your walls and ensure even adhesion. You'll also want to spot treat the nails and drywall tape to ensure even base coloring to prevent the different colors from showing through the wallpaper.
• Oil or Gloss Painted Walls must be sanded to until dull to provide a good surface for the wallpaper to adhere to. A coarse sandpaper should be used with an acrylic primer/sealer.
• Flat Latex Painted Walls should be treated with a pigmented, arcylic primer/sealer.
• Existing Wallpaper.  It's always better to remove existing wallpaper if possible before installing your new paper. If there are any loose areas, the new wallpaper cannot be installed successfully. If the old paper is in good condition and removing might cause damage to the underlying wall, you may choose to hang your new paper over the old. To prepare this type of wall, spackle and sand every seam of the old wallpaper and reseal each seam or tear with seam adhesive. Test the color of the old paper to ensure it will not bleed through to the new (ammonia & water can be used to test the color - colorfast inks will not react to the solution).  If the color test fails, first apply a stain killer primer/sealer and follow up with an acrylic primer/sealer. If the color does not react, a pigmented acrylic primer/sealer can be used. Use a vinyl over vinyl adhesive to install the paper.
• Textured Walls. Any wall surface must be sanded smooth before wallpaper installation or the papers will stick only to the higher outer surface as well as cause problems with the seams. An electric belt sander is often best for this. Use an acrylic primer/sealer once the walls have been sanded smooth. Select your wallpaper carefully for this type of wall. You will not want to use a smooth or shiny paper as it will accent the underlying textures.
• Brick & Block Walls can be wallpapered but special preparation must be done. Also, most of these products have a natural uneven surface which cannot be altered and these surface distortion will be visible when papered. For brick & block, moisture must be controlled before considering this type of project. Use a heavy wallpaper liner to help provide a smooth surface and an acrylic primer/sealer. Bricks with a grainy surface are not suitable for wallcovering as they provide poor adhesion.
• Ceramic Tile is generally found in bathrooms or kitchens and must be thoroughly cleaned to remove any grease buildups. A heavier wall liner should be used with an acrylic primer/sealer.
• Plywood walls can be covered with wallcoverings if properly prepared first. Use an oil-based primer/sealer to protect the natural grain of the wood from raising as the result of moisture from the adhesive.  Any natural texture from the wood grain should be sanded.  
• Paneled walls can be wallpapered with the use of spackle to smooth out the seams.  The use of a liner may be considered depending on the condition of the paneling.  An pigmented acrylic primer/sealer should be used before installing the wallpaper.
• Plastered walls must be completely dry before installing wallpaper to allow the plaster time to cure. A strong vinegar/water solution should be used to neutralized any alkaline areas of the wall. Cracks must be carefully considered. Small hairline cracks can be spackled. Larger cracks should use mesh to repair the crack. Once dry, an acrylic primer/sealer should be used before installation.