In general, for window plaster, as for all materials mixed with water, extremes in humidity, temperature, wind and drafts can cause damage. Processing on exterior walls is often planned within a tight time frame. A later sloping or cracked external plaster then opens a new time window.
Influences on the drying behavior
When a structural plaster is applied, set and dried out on the facade, physical processes arise that form a durable, hard layer from a movable and viscous mass. In a somewhat simplified way, water bound in the plaster escapes and leaves a hard shell made of lime, sand, cement and other additives.
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The duration of the evaporation of the water is decisive for this process. It is influenced by the following external factors:
- The water evaporates faster when it is warm
- When it is cold, the water evaporates more slowly
- Evaporation occurs when it is frosty
- Evaporation slows down when the air humidity is high
- If there is a strong draft, the rate of evaporation increases
The most important requirement is timely setting and drying
Plaster is designed to show its physical bond and stability in a "healthy" period. After the correct application of plaster, the fresh wall covering needs the following conditions:
- Ideal outside temperatures are moderate between ten and 25 degrees Celsius
- Possible minimum and maximum temperatures are five and thirty degrees
- Direct sunlight should be avoided (shading)
- Avoid high humidity of around 75 percent (rain, fog, dew)
- Avoid times of strong winds and gale force gusts
Drying time and the alternative scratch plaster
In transitional periods such as spring and autumn, the rule of thumb of the drying time for window plaster should be observed. Around 24 hours are calculated for each millimeter of plaster thickness. As the plaster dries from above, the surface appears dry more quickly.
For this reason, too, when it comes to the question of whether scratch plaster or glass plaster is better suited for an external facade, the decision is often in favor of scratch plaster. A facade with scratched plaster can defy unexpected weather changes faster than glazed plaster.Tips & Tricks If you are plastering during a phase in which drying is slower, a downpour can simply wash the plaster off the wall even after a few days.