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How to Size Walls for Wall Coverings

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. The National Guild of Professional Paperhangers recommends you 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.

Preparation

Use a broad-bristled brush to apply size to a wall.
  1. 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.

  2. 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.

  3. 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.

  4. Scrub over the wall with a dry scrubbing brush to remove any dust or debris.

Mixing and Application

  1. 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.

  2. 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.

  3. 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.

  4. Paint the size mixture over the wall surface using a broad-bristled brush.

  5. 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.

Warning

  • Turn off the electrical supply before sponging down the wall to reduce risk of electrocution.