Backing your favourite posters with linen will preserve them for years to come. Linen backing does take some practice and skill, so don't try mounting collectable posters or posters with sentimental value until you've successfully mounted a few practice pieces. You don't need to purchase materials from special supply sources; the "linen" used for mounting many posters is actually a light canvas, which you can find at most craft and fabric stores. Hardware stores and international grocery stores will carry the other items necessary for backing posters.
Machine wash your linen or light canvas backing fabric to remove the sizing. Don't add any fabric softener to the wash. Allow the fabric to air dry.
Make a batch of wheat paste. Combine 1/4 cup wheat starch with 3/4 cups water and allow the mixture to sit for at least 30 minutes. Cook the soaked mixture over medium heat, stirring constantly. When the paste looks translucent, remove it from the heat and allow it to cool into a semisolid form.
Strain the semisolid paste through a fine mesh strainer and add enough water to create a thin, lumpless adhesive.
Place the washed backing fabric on a fine-mesh screen with a wooden frame. Screens usually used for silkscreening work well as long as your poster will fit within the frame.
Dip a paint roller in the wheat paste and apply a thin layer to the fabric. Smooth out any wrinkles as you apply the paste.
Set the frame and fabric aside and cover your work surface with a sheet of mylar that measures at least 6 inches larger than your poster.
Place the poster face down on the mylar.
Spray the back of the poster with a mist of distilled water to relax the paper.
Dip a paint roller in the wheat paste and apply a thin coat of paste to the poster's back. Work over the entire surface of the poster, carefully smoothing any wrinkles and distributing some paste around the poster's edges.
Pick up the mylar and paste-covered poster and place it face up on the linen backing. Adhere the upper corners first and allow the rest of the poster to fall into place.
Carry the linen, poster and mylar-covered screen to your work surface and lay it flat.
Smooth out any creases or air bubbles in the poster. Leave the mylar sheet in place and use a brayer to adhere the poster to its linen backing and work out any air bubbles.
Remove the mylar sheet and allow the linen-backed poster to dry.
Peel the dry poster and fabric from its screen and trim excess fabric from around the edges. Use a utility knife and a straightedge ruler to make precision cuts.
Look for wheat starch at your local Oriental market. If you need to find large screenprinting screens, Standard Screen Supply Corp carries screens as large as 36-by-36 inches.