Test it first – then prepare it
It is no coincidence that one of the rules of wallpapering is: “The person applying the wallpaper is obligated to test the wall surface for suitability.” Whoever fails to do this is liable for any arising defects. Mistakes in wallpapering often have nothing to do with defective paper or application errors. Instead, the cause is frequently insufcient testing or improper preparation of the wall surface. It is best to proceed methodically.
To determine the alkalinity, moisten the plaster with neutral water and test using indicator paper. In the event of high humidity, for natural material or metallic wallpaper, pH values of more than 8 can be problematic. Discolouration may result.
An insulating primer coat usually helps, which, to be on the safe side, is covered with non-woven lining paper. In any case, allow fresh plaster to dry sufficeintly.
Mould stains occur in high humidity, a lack of airing, insufcient heating and a lack of sunlight.
Thoroughly remove mould and treat area with a fungicidal solution; if necessary, apply a base coat of fungicide. But be careful: fungicides are harmful.
Measuring humidity with a hydromat only supplies relative results. More precise but more complicated are the calcium carbide method (CM device) and the Darr method. For both, a material sample is taken.
Allow the walls to dry. If time is short, use an industrial dryer.
Superficial discolouration due to the seepage of soluble components, of orescence in the form of yellow salts or corrosion stains, e.g. due to insufficiently covered rebars.
Eliminate the source of the moisture damage. Allow the wall surface to dry. If there is efforescence, brush off the salt. If there are corrosion stains, paint the metal components with a rust-proof paint. In the case of discolouration, insulate with a sealer.
A scratch test reveals insufficient stability of the wall surface.
Remove loose parts and, if necessary, set the wall surface with a hydrosol primer.
Chalking of the wall surface can be identified through a simple wipe test; the surface rubs off.
Remove the chalking surface and, if necessary, set the surface with a primer.
A tear-off test using adhesive tape quickly reveals any old paint layers that are not set.
Completely remove old loose layers of paint before wallpapering.
If, during a spray test, water beads on paperable concrete or cement surfaces, this indicates formwork release oil residue or a sinter layer. Usually, the surface will be shiny and not very absorbent.
Even out surfaces having varying absorption characteristics using a primer coat; if necessary, first lightly sand the entire surface.
1. Shrinkage, hairline or stress cracks are short and relatively evenly distributed.
2. Cracks originating from the plaster base can be identfied by the way they run (see left).
3. Constructional cracks are caused by movements in the structure and are deep.
1. Even out the entire area using a fibrous dispersion filler.
2. Reinforce with non-woven or woven material.
3. Permanent bridging of cracks is only possible when the walls have settled.