Sponge down the wall with water and a household detergent to remove dirt and grease. Rinse the detergent away with clean water. Allow the wall to dry.
Fill any cracks in the wall surface with a plaster filling compound, using an old table knife to get the compound into the crack. Smooth the surface of the repair over with the flat of the knife blade. Allow the compound to dry, then sand it flat, using sandpaper.
Paint over any spots where there has been mold on the wall, using a paintbrush dipped in a half-and-half solution of water and household bleach. Let the bleach solution dry.
Scrub over the wall with a dry scrubbing brush to remove any dust or debris.
Mixing and Application
Tip the sizing powder into a bucket. Follow the manufacturer's instructions for the quantity you require. As a guide, you will need around 4 1/2 ounces of sizing powder to make 12 pints of size.
Pour a little warm water into the bucket. Stir the water well into the powder using a stick, until you have made a thick paste.
Keep adding water, a little at a time and mixing well, until you have approximately 12 pints of whitish liquid, about the consistency of thin latex paint. Leave the bucket of size to cure for 45 minutes, stirring occasionally, to assure the powder has dissolved completely.
Paint the size mixture over the wall surface using a broad-bristled brush.
Allow the size to dry. This should take about 30 minutes. Apply the wallpaper. The size prevents the porous plaster absorbing too much paste, ensuring a strong bond between the paper and the wall. Sizing a wall also makes it easier to slide the pasted wallpaper into place.
Things You Will Need
- Household detergent
- Plaster filling compound
- Table knife
- Scrubbing brush
- Household bleach
- Sizing powder
- Broad-bristled brush
- If you cannot obtain sizing powder, use ordinary wallpaper paste thinned with water until the mixture resembles thin latex paint.
- Size is traditionally used only on plaster. On other materials such as plasterboard -- also called drywall, sheetrock, or gypsum board -- apply an acrylic primer instead.
- Avoid sizing powder made from cornflower and starch, as it can encourage mildew. Instead choose sizing powder made from pine flower and cellulose, or clay-based adhesives.