Lesson 12. Rules for Applying Plaster Mixes in Wall Preparation

In the previous two lessons, we examined the sequence of steps for preparing walls to any quality level Q in rooms with dry, medium, and humid microclimates. This structure clearly shows how plasters and putties, applied step by step, gradually increase the flatness and smoothness of the surface.

It is important to note that construction traditions vary across countries. In some regions, such as the United States and Canada, concrete, brick, and block walls are less common in residential construction, where wood‑frame building and drywall dominate.
Precisely because these mineral bases are encountered less frequently, knowledge of their preparation is even more critical. The rarer the material, the greater the chance that a craftsman accustomed to drywall may lack experience and make mistakes when working with brick or block walls.

As decorators applying the final decorative plaster layer, we must understand these rules thoroughly. If we see obvious preparation errors, it is better not to risk applying décor on poorly prepared walls. Decorative plaster relies on the quality of the base: if the preparation is flawed, the finish will not be durable and will quickly lose its beauty.

In this lesson, you will learn the rules for working with construction mixes and the factors that must be considered during wall preparation. Violating these rules can lead to serious defects and, as a result, require a complete redo of the entire preparation process.

Priming the Surface and Microclimate

As discussed earlier, the preparation of different mineral bases and the structure of the so‑called “layer cake” — the combination of all material layers — depends on the properties of the base. For example, the first step in preparing brick walls differs from foam block walls: foam block requires double priming.

These differences are explained by the properties of the materials used for rough walls, as well as the characteristics of the construction mixes applied to them. Each base has its own features that directly affect adhesion and the durability of the finish.

For plaster to perform its functions properly, it is important to consider not only the application technology but also drying conditions: air temperature, humidity, and the absorbency of the base.

• Too fast drying (high temperature, low humidity) causes cracking due to uneven evaporation of moisture.
• Too slow drying reduces adhesion and creates conditions for mold and fungus.

Highly porous surfaces quickly draw moisture from the plaster, causing premature drying and cracks. To prevent this, impregnating and strengthening primers are used. They not only remove dust but also reduce absorbency. Normally one layer is enough, but highly absorbent bases require at least two.

The method of primer application is also important. A roller with long pile is optimal — it distributes the primer evenly and penetrates the surface well. Spraying is less effective, as it is harder to control uniformity, which can lead to excess or insufficient coverage.

Thus, proper plaster drying depends not only on air temperature and humidity but also on the absorbency of the base. Freshly applied material simultaneously releases moisture into the base and into the air. Therefore, it is essential to follow manufacturer recommendations and carefully prime porous surfaces. The same principle applies to decorative materials.

Combining Different Base Materials

In construction, it is common to encounter walls made of different materials. For example, load‑bearing walls may be concrete or reinforced concrete, while partitions may be brick, foam block, aerated concrete, or expanded clay block. Sometimes existing walls are extended with another material.

At the joints of dissimilar bases, cracks often form after plastering due to differences in shrinkage, strength, and absorbency. To avoid this, such joints are reinforced with facade plaster mesh, usually in one or two layers.

Example: If part of a wall is concrete and the extension is foam block, the joint is primed (concrete — one layer, foam block — two layers). A 5 mm thick, 20 cm wide strip of plaster is applied, and a 15 cm wide mesh is embedded so that its center lies exactly on the joint. Without waiting for drying, a second 5 mm thick, 35–40 cm wide strip of plaster is applied, embedding another 30 cm wide mesh. This double reinforcement significantly reduces the risk of cracks.

The same principle applies to corners and to window and door openings, which accumulate stress and are prone to cracking. Double reinforcement with facade mesh prevents deep, hard‑to‑repair defects.

Interaction of Gypsum and Cement

Another critical aspect is the interaction between gypsum and cement. As mentioned earlier, when applying gypsum plaster to concrete or reinforced concrete, a bonding primer (betonokontakt) must be used.

Gypsum sulfates react with tricalcium aluminate in cement, forming ettringite. This mineral expands, creating pressure between plaster and base, leading to cracks and detachment.

Mistakes also occur when cement plaster is applied but guide profiles are fixed with gypsum “dots.” Gypsum remains inside the cement layer, provoking ettringite growth. Over time, bumps appear under wallpaper or decorative finishes.

To avoid this, guide profiles should be fixed with fast‑setting cement‑based compounds, not gypsum. The same applies to floor leveling and tile installation: gypsum residues in cement layers eventually cause deformation and detachment.

Building Settlement

Finally, building settlement must be considered. Settlement is the gradual deformation of structures due to drying, compaction, temperature changes, and loads. It is most intense in the first years after construction.

Therefore, plastering in new buildings should be done after major settlement has occurred. To reduce cracking, plaster is cut along wall‑to‑floor and wall‑to‑ceiling joints while still fresh, creating a thin gap that relieves pressure.

Even with reinforcement and expansion joints, it is recommended to wait at least 1 year (1.5–2 years for massive structures) before applying final finishes. This way, settlement cracks appear early and can be corrected before expensive decorative work.

Conclusion

Congratulations! You have studied the fundamentals that most craftsmen overlook at the very start — mistakes that can cost reputation and money.

You now understand the mechanics of applying base mixes and the Q1–Q4 preparation system for the most common bases. You already see surfaces differently and possess knowledge not available in my YouTube videos.

But this is only the foundation. In the main program of Module 1, we cover five more critical topics:

• Lessons 13 & 14: Drywall — how to avoid cracks on the most demanding base.
• Lessons 15 & 16: Defect analysis — how to “treat” problem walls and avoid responsibility for others’ mistakes.
• Lesson 17: Selection algorithm — how to choose the right material and technique for wall quality.

After Module 1, we move to the next levels of professional approach:

• Materials — understanding composition, behavior, and limitations of mixes.
• Tools — mastering the logic of tool interaction with decorative materials.
• Coloring — learning color theory and achieving precise shades.
• Practice (15 techniques) — from silk and Venetian plaster to concrete and travertine, with full understanding of layer structure and sequence.

You have completed 12 lessons, and I see your drive for knowledge. My goal is for this course to become a true investment, bringing results from your very first projects.