You must size a plastered wall to get the best results when hanging wallpaper. "Size" is the name given to the liquid you paint over bare plaster before applying a wall covering. You need to do this preparation for two reasons: to help the covering bond well to the wall and to ensure the wall remains undamaged when the time comes to remove the covering. Sizing a plastered wall is quick and easy to do, if you follow a few simple steps.
Sponge down the wall with water and a household cleaning fluid 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 mould 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 128 g (4 1/2 oz) of sizing powder to make 5.7 litres (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 5.7 litres (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.
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 apply an acrylic primer instead. Avoid sizing powder made from corn flour and starch, as it can encourage mildew. Instead choose sizing powder made from pine flower and cellulose, or clay-based adhesives.
Turn off the electrical supply before sponging down the wall to reduce risk of electrocution.