Sizing

     Sizing is a very important part of the wallcovering job.  Sizing is painting your walls with a product that draws water from your paste and helps your material adhere better.  Boxes of powdered sizes are hard to find so you might try painting the wall with a thinned solution of paste.  But do not use flat latex paint for a sizing. 

     If you hang wallpaper on a flat latex paint, the sponge-like surface of the latex will soak all the glue from your wallcovering and make the product very difficult to position on the wall.  This also causes the paste to dry too quickly and weakens the bond.  This will usually manifest itself as loose spots and shrinking back at the seams.  This is especially common with prepasted wallcoverings that were installed on flat latex paint with no sizing.

      And this is only half the problem.  Because latex is very porous, where there was plenty of paste, the paste will soak through the paint and bond deep into the surface, usually gypsum board.  This leaves you with a monumental stripping job if you ever want to change wallcoverings.  The wallcovering will actually be bonded so tight that when you try to strip it, it will pull the gypsum paper off your drywall surface and leave you with an expensive and time-consuming repair job.

      Prep coats are often confused with sizing.  Prep coats are products that try to do two jobs at once.  Most of them are acrylic based so they will dry hard like a primer coat.  They are also very porous so they behave somewhat like sizing too.  This has made prep coats very popular across the wallcovering industry.  There are many brands that come in clear or pigmented bases that can be colored. 

     Most prep coats are porous enough to serve as a sizing material, but not all of them are hard enough to provide for easy stripping in the future.  You can increase the likelihood of easy stripping in the future by not thinning the prep coat and letting it dry for a day or two.  This extra drying time, called 'curing', allows the prep coat to get harder and more impervious to water from the paste.

     Pigmented wall preps are also good over new sheetrock because they remedy the contrast between the color of the drywall (gray) and the joint compound (white).  This color difference might show through some thin or white background wallcoverings.

      Most preps have mildew killers already in them.  These are necessary for high humidity areas like showers and even tropical locations.  If you have serious mildew problems, you can add mildew killer to your paste as well.

     It is important to remember that sealing your wall for future stripability with a hard coat like a primer or sealer is different than sizing for better adhesion.  In fact, some of the new oil primers are so dense that they reduce the ability of the paste to adhere to them.  This is why you will often need a two step process to be in good shape.  A primer coat for wall protection and a sizing coat for adhesion.