Why does the old wall covering need to be removed?
It is usually preferred to remove the existing wall covering. The moisture content in the paste, as the new wall covering is drying, may bubble or lift the old wall covering. In some cases where the wall covering is very tightly secured and removing it could cause considerable damage to the wall surface, an alternative might be to remove any loose areas to spackle and repair any irregularities, and to use a primer sealer over the existing wall covering.
