Wall Skimming

It's tempting to skip it often, as the process is dusty and labor-intensive, requiring skill and expertise.

Let’s take a closer look at the technique of wall leveling with skim coating, types of skim coating materials, and various application methods depending on the planned final finish.

Types and Composition of Skim Coatings

The main materials used in making skim coating compounds are gypsum, cement, and latex. A critical parameter for skim coating is the grain size—smaller grains allow for thinner application layers. The finest grains are found in latex-based skim coatings, while the coarsest are in cement-based ones. This directly affects their applications.

Latex and acrylic skim coating compounds are used for cosmetic preparation of walls prior to painting. For example, they are applied to gypsum board not to level the plane—since it’s already flat when new construction uses gypsum board or gypsum fiberboard—but to cover joints and screw points, preventing cracks and visible screw heads through wallpaper or paint in the future.

Gypsum-based skim coatings with coarser grains are applied in layers up to 5 mm over several passes and require sanding to achieve a perfectly flat surface. These coatings are used for leveling older surfaces made of brick, gas silicate blocks, and similar materials.

Cement-based skim coating, in addition to its very coarse grain allowing for thicker application, has a key advantage over the other two types: it is water-resistant. Therefore, it can be used for external facade finishing of buildings.

Step-by-Step Guide to Applying Skim Coating

Any skim coating has poor adhesion (ability to stick) to many substrates (brick, cellular concrete, monolithic structures, etc.). Therefore, before application, the wall must be primed with deep-penetration primer. This solution improves adhesion between the coating and wall and prevents mold and mildew formation.

Next, all complex areas on the wall (joints of Gypsum Board sheets, possible dents, hollows, and other spots where thicker layers will be applied) are reinforced with reinforcing mesh—serpianka. This consists of thin glass fiber threads woven together with an adhesive layer on one side for secure attachment to the wall. The role of serpianka is especially critical at Gypsum Board joints, as even properly applied skim coating will inevitably crack at these joints, and a dark crack will appear in the final finish.

If the wall is uneven, the simplest way to restore a flat surface is by setting up guide profiles (masons). There are many methods, but the easiest is to screw in screws at the four corners and stretch (as tightly as possible) nylon string or twine. The distance from the string to the wall can be adjusted by tightening or loosening the screws. Then, guide profiles are installed along the string, fastened mechanically (with screws) at 1–1.5 meter intervals.

Preparation of adhesive compound. Some skim coatings (e.g., latex-based) are supplied ready-to-use in plastic buckets or bags. Gypsum and cement-based coatings require prior preparation, involving mixing dry powder with water and stirring evenly until a flexible, paste-like consistency is achieved. It’s best to do this in special plastic containers, using a drill with a special mixing attachment.

Then, the first layer of skim coating is applied. It’s most convenient to use a wide trowel (metal or plastic), spreading the compound uniformly from top to bottom or left to right. The layer thickness decreases as the angle between the trowel and wall approaches 90 degrees. Avoid over-thick layers—excessive thickness may cause cracking or even delamination during drying.

After the first layer is completely dry, it must be sanded to remove visible irregularities and roughness. Then, the final second layer is applied. Its thickness should not exceed 1–2 mm.

The final step is final sanding (polishing) of the dried skim coating layer using very fine sandpaper (grade 180, 200, or finer). This can be done manually with a sanding block or using a modern electric sanding machine.

After the entire skimmed area is fully dry, it must be cleaned of dust, and it will be ready for further finishing—painting or wallpapering. Typically, a person who has never done skim coating by hand finds it difficult in the first 2–3 square meters, but after that, the process becomes easier and more successful.