Lesson 8. Quality Criteria Q1, Q2, Q3, Q4In the previous lessons, we studied the logic of how the Q1–Q4 surface preparation quality system works and its role in choosing decorative materials, as well as why it is important for effective interaction between the decorator, the client, and the craftsmen preparing the walls. In this lesson, you will learn the specific requirements of each level in the Q1–Q4 system and the key differences between Q3 and Q4, which are especially important for applying thin‑layer decorative coatings.
We will start with level
Q1. Using it as an example, we will examine how each criterion is checked.
Q1 Surface Requirements- Deviations in vertical alignment up to 3 mm per 2 linear meters are allowed. To check this indicator of wall flatness, a 2‑meter straightedge is used. It is applied across the surface, and if gaps appear up to a maximum of 3 mm, such flatness corresponds to the lowest quality level — Q1.
- Visible indentations, scratches, gouges, and tool marks (up to 3 mm deep) are allowed. Inspection is carried out visually under normal diffused lighting from a distance of 1–1.5 m. Side or raking light is not used so as not to exaggerate defect visibility. A ruler may be used to clarify defect depth.
- Local plaster bulges (up to 2 mm high) are allowed. Inspection is visual under normal diffused lighting from a distance of 1–1.5 m. Side light is not applied. If necessary, a straightedge or flat rail is used to confirm that the bulge does not protrude more than 2 mm relative to the main plane.
- Shadows from side lighting are allowed. Any defects permissible at Q1 will create light‑shadow patterns under raking light. This is natural and considered normal for this level. Therefore, using raking light to check Q1 quality makes no sense — its purpose is to reveal the smallest flaws, which at this stage are not considered defects.
Q2 Surface Requirements- Deviations in vertical alignment up to 2 mm per 2 linear meters are allowed.
- Visible indentations, scratches, gouges, and tool marks (up to 1 mm deep) are allowed.
- Local plaster bulges (up to 1 mm high) are allowed.
- Shadows from side lighting are allowed. For Q2 preparation, raking light is not used, since this level does not involve identifying the smallest defects. It is sufficient to check for vertical deviations or scratches, indentations, or bulges visible to the naked eye.
Q3 Surface Requirements- Deviations in vertical alignment up to 1 mm per 2 linear meters are allowed.
- Visible indentations, scratches, gouges, and tool marks are not allowed.
- Local plaster bulges are not allowed.
- Shadows from side lighting are allowed. It is important to understand that Q3 is a very high level of surface preparation, which is not easy to achieve. This level permits the application of most thin‑layer decorative materials, and therefore it is crucial to eliminate the coarse defects typical of Q1 and Q2.
To achieve Q3 quality, craftsmen preparing the walls must use raking light. Without it, it is almost impossible to detect and correct coarse defects in time. If raking light is not used during putty application and sanding, there is a high probability that flaws corresponding to Q2 or even Q1 will remain.
Using a raking lamp during inspection allows such defects to be quickly identified and corrected before the decorative layer is applied. This ensures that the base truly meets Q3 requirements and is ready for most thin‑layer decorative materials.
In this context, permissible shadows refer to shadows from surface irregularities corresponding to the first criterion — up to 1 mm per 2 linear meters. Such minor deviations do not affect the aesthetic appeal of most thin‑layer decorative materials and are considered acceptable.
It is important to distinguish: if a scratch or indentation is visible only under raking light, this does not mean that preparation fails to meet Q3 criterion 2. Here it is important not to impose excessively strict requirements on minor traces that appear only under raking light but remain invisible under normal diffused lighting from 1–1.5 m and do not affect the final result.
5. Visible scratches from sanding abrasives (up to 0.3 mm deep) are allowed. On uniform white putty, such defects are often invisible under normal lighting and appear only when using side (raking) light. This is normal for this level of preparation, since after applying most coatings such scratches will not be noticeable.
Q4 Surface Requirements- Deviations in vertical alignment up to 0.5 mm per 2 linear meters are allowed.
- Visible indentations, scratches, gouges, and tool marks are not allowed.
- Local plaster bulges are not allowed.
- Shadows from side lighting are not allowed.
- Visible scratches from abrasives after sanding are not allowed.
Q4 is the highest level of surface preparation quality, achievable within reasonable timeframes only by experienced craftsmen specializing in wall preparation, which makes this level the most expensive. Therefore, it is important to clearly understand when Q4 is truly necessary, and when its use would lead to unnecessary costs of time, materials, and money. The answer lies in understanding the difference between Q3 and Q4.
If the differences between Q1, Q2, and Q3 are obvious — each subsequent requirement is noticeably stricter than the previous one — then Q3 and Q4 are largely similar. At both Q3 and Q4, the surface must be free of visible defects: indentations, scratches, gouges, tool marks, or local plaster bulges are not allowed. These flaws are checked by ordinary visual inspection under diffused lighting from a distance of 1–1.5 m.
The difference lies in the reaction of the surface to side lighting. Unlike Q4, at Q3 it is acceptable that under strong side lighting along the wall, shadows from minor irregularities may appear. These shadows are created by such small indentations, scratches, gouges, tool marks, or plaster bulges that under diffused light they are simply invisible at Q3.
In real conditions, of course, no one illuminates a wall with a special raking lamp, but similar sources of side light are common. This is especially noticeable in rooms with windows facing the sunny side: at noon, rays passing through the window glide along the wall and highlight even the smallest irregularities. Artificial sources can create the same effect — wall‑mounted lights or powerful LED strips placed along the wall. In such conditions, any flaws become especially noticeable, and the use of thin‑layer decorative materials requires Q4 preparation, since Q3 will reduce the aesthetic appeal of the coating.
It is necessary to plan in advance how the room will be lit and how different light sources will interact with the wall finish. Sometimes it is enough to bring only one or two walls, or even specific sections, to Q4 — precisely those exposed to side lighting — while Q3 preparation is sufficient for the rest.
It is also important to understand which thin‑layer material will be applied. Glossy paints and ultra‑thin coatings with metallic effects can be compared to raking light. Even without side lighting, they emphasize all scratches, marks, and the smallest irregularities. Defects that remain invisible at Q3 and do not interfere with most thin‑layer decorative materials become obvious on glossy finishes. Therefore, such demanding coatings require Q4 preparation.
High light reflection is characteristic of only a few thin‑layer materials. Mainly these are acrylic paints with high gloss, as well as decorative paints with metallic effects (gold, silver, steel, pearl). For such coatings, it is important that light reflects evenly, without the slightest distortion.
Usually, such paints are applied by spraying, since this method allows the material to be distributed as evenly as possible and achieve a perfect shine across the entire surface. Roller application is rare: it leaves a slight texture that disrupts the mirror‑like gloss effect.
Decorators generally do not apply such materials to create perfectly smooth glossy surfaces — this is more the work of professional painters specializing in airless spraying. In our field, Q4 may be required due to side lighting, for applying decorative sand with a metallic effect or certain types of “wet silk.” Essentially, decorative sand with a metallic effect is the same metallic paint (gold, silver, steel, pearl), but with filler added in the form of grains or glass microspheres. Examples include
Lucetezza oro and
aluminio by Decorazza, or similar products. When applied with a brush, areas remain between the grains where the smooth metallic paint layer is visible. This layer reflects even diffused light well, highlighting small base defects. Because of this metallic effect, such decorative material requires Q4 preparation.
Maximum base preparation is also needed for applying certain types of decorative plaster “wet silk.” These are variants that create ultra‑thin coatings with strong shine. Examples include
Lisse and
Soie brillante by Vincent,
Seteria by Ticiano, and
Seta by Decorazza.
Understanding the relationship between light, surface quality, and material gloss gives the decorator not only the ability to correctly select thin‑layer coatings for specific conditions, but also opens the way to creating expressive artistic effects. For example, if the goal is to emphasize the volume of textured décor, side lighting or glossy materials will enhance the depth of relief. And when the task is to visually smooth the surface, diffused light and matte coatings can be chosen. This is one of the most effective tools for controlling the perception of volume in textured decorative plaster.
In this lesson, you studied the specific requirements of each level in the Q1–Q4 system, as well as the key difference between Q3 and Q4. In the next lesson, we will discuss in detail the materials used in surface preparation. All this information forms the foundation of professional decorator’s work and is very important. We remember that decorative plaster is a finishing material intended to give the surface aesthetic appeal. With each lesson, you are uncovering more and more of the essence of this definition, which will ultimately allow you to achieve high‑quality results and maximum aesthetic appeal in the décor created by your own hands.
In the next session, we will delve into the materials used for base preparation, to better understand the challenges that may arise when working with decorative plaster.
