If blisters appear as the wallcovering lengths start to dry out, it usually means there are air pockets underneath. Ease the paper away and brush it down again, expelling the air as you do so. If they persist, it could mean that the wallcovering has not had time to absorb sufficient moisture from the paste - indicating that the lengths need to be left longer after pasting so that they become "limp" before hanging.
The reason is that the fibres, after wetting with paste, start to expand. If the wallcovering is put onto the wall before this has finished, the fibres continue to expand, causing blisters to form. Sometimes these flatten out after the wallcovering has dried out - but not always.
Using a wooden or rubber roller to "iron out" a blister is rarely satisfactory. Rollers are mostly used on speciality wallcoverings to press down particularly stubborn joints, and in these cases, a piece of paper should be sandwiched between the roller and the wallcovering so there is no direct contact and friction.
Other causes of blisters are:
- Too thin a paste, giving lack of adhesion in places and/or lack of penetration into the wallcovering.
- Inadequate pasting, allowing areas of wallcovering to be missed and left dry.
- Hanging onto a hard, impervious wall which provides no key for the paste. This means the paste has to dry through the front of the wallcovering, not necessarily drying evenly.
- Paste too thick and overloading the wallcovering with paste, resulting not only in blistering but also creasing, stretching and flattening of the emboss.
