A good surface is essential for a perfect result. All too often people do not consider this or are too nonchalant about the surface. The results can be disastrous. When painting, preparation (sanding, washing down, plastering, fixing, etc) can easily take up half of the time spent. When wallpapering however, people often think that problem areas will be covered up anyway. So many problems could be avoided by just paying attention and taking a few simple steps. The key word is prevention as it is often impossible to remedy or repair disappointing results caused by poor surfaces.
What is a good surface?
• Dry:
If there is any moisture left in a newly plastered wall, it will be necessary to wait until it has dried. To accelerate the drying process, we recommend that you keep the room warm and ventilate it regularly. Rising damp or damp caused by construction errors must be tackled at the source of the problem. If this is not possible, the surface cannot be considered as a good surface. If the plaster has patches of mould or dark spots, it must not be papered over. Even if the patches look dry and do not feel damp, it is possible that damp will percolate through if there is a lot of rain.
• Solid:
A surface must be strong enough to bear the wallcovering. This means that the plaster must be able to bear the tension that occurs during drying. This is often a problem with old, crackled layers of plaster (grey and white) in particular.
Tension does not only occur when the wallcovering is drying. In autumn, when the heating is turned on again, the sharp drop in humidity in the room may increase tension in the wallcovering. Powdery surfaces must be fixed. To do so, use an adhesive that penetrates deep into the surface. The product must never form a film on the surface that would seal it completely. If a surface is giving you doubts, or if you have a strong suspicion that it might react, we recommend that you use a fixative, which you may dilute.
Old layers of paint that do not adhere well or evenly may also cause problems. Dry plastering (gypsum boards) may cause problems when removing wallcoverings. These boards are covered in a layer of cardboard that is inclined to come loose in places or to become weak when dampened. In this case, we recommend a synthetic fixative that permeates the surface. This will prevent problems when wallpapering and will make the wallcovering easier to remove at a later date. If you use a pigmented product in this phase, you have the added advantage that the surface will be an even colour.
• Slightly absorbent:
The adhesive must be able to permeate the wall. If the plaster is completely sealed (by paint, for example), and therefore is unable to absorb adhesive, it will be impossible to achieve the right connection between the surface and the wallcovering. If the surface is too absorbent, it is best to pre-paste it or in extreme cases, to fix it (caution: use a product with a penetrative effect). If surfaces are too impermeable, they must be roughened. One possible solution is an extra under layer with non-woven lining (such as Patent Vlies).
• Smooth:
The plaster must not have any visible irregularities. The less relief in the wallcovering, the smoother the surface must be. This applies even more so to glossy or silk glossy uni wallcoverings. Any irregularities will be especially obvious in slanting sunlight.
Uneven patches are not always caused by irregularities in the plaster – even small hairs from a wallcovering brush may show up on glossy wallcoverings.
• Clean:
Papering over existing wallcoverings is just asking for problems. If the seams of the old wallcovering do not join perfectly, this will show up in the new layer of wallcovering. In some cases, the old wallcovering may come loose. Dirt in the old wallcovering may also cause stains to show through in the new wallcovering.
• Neutral:
Freshly applied plaster and cement are often highly alkaline (pH>10). This decreases as the plaster dries out. You can use a pH indicator stick to check if a wall is alkaline. The wall should be dampened with distilled water at various places, after which the indicator paper is applied to the patches. The resulting discolouration can be compared to the colours on the pH meter. A value of 7 or more indicates an alkaline or acetous surface. In this case, the surface must be neutralised.
There are neutralizing agents to neutralize alkaline surfaces. They are sold in the shops as either a powder or liquid. A neutralized surface is harder, denser and less absorbent. Also watch out with repairs to the plaster. Some plasters are very acidic. If they are not isolated
sufficiently there is a risk that discolouration will occur when you are wall papering. This is more likely to occur with vinyl wallcoverings. If this wallcovering dries more slowly, there is a danger that more alkalinity will be generated.
Metallic wallcoverings or wallcoverings with natural fibres are significantly more susceptible to alkalinity. Metallic wallcoverings may not be used on an alkaline or acidic surface.
• Uniform light colour:
Colour differences in the wall may be visible, especially with thin, light-coloured non-woven wallcovering; therefore the surface should be a uniform light colour.
Summary: A suitable surface must comply with the following requirements:
• Dry
• Solid
• Slightly absorbent
• Smooth
• Clean
• Neutral
• Uniform light colour
