Priming the walls before plastering – help in choosing the right composition

There’s a crucial step you shouldn’t omit before plastering your walls: priming. It’s not just an extra chore to prime the walls before applying plaster; it’s essential to making sure your plaster job looks good and lasts for many years.

With so many options available on the market, selecting the best primer can be a little confusing. Every kind of primer is made to perform best on particular kinds of plaster and wall surfaces. If you do it correctly, your plaster will adhere more readily, dry more evenly, and be more resilient to problems like peeling or cracking.

We will walk you through the fundamentals of choosing the best primer for your walls in this article. We will assist you in determining which primer composition is most appropriate for your project, regardless of whether you are working with new or old, porous or non-porous surfaces.

Do I need to prime the walls before plastering?

Why are primers used?

• strengthening loose bases (binding dust);
• increasing adhesion;
• sealing pores;
• protection from microorganisms.

Put another way, while primers serve a variety of functions, their primary goal is to guarantee a solid, long-lasting interlayer connection between building materials. When materials can be readily connected to one another without the need for middlemen, primers are not required.

The amount of plaster mixtures used for leveling decreases when impregnations are used.

Let’s examine the potential issues that the primer aims to resolve:

1. A loose porous base that has low strength, dusty (staining fingers when touched) needs to be strengthened. If you apply a plaster composition to it, then, having wetted and bound the upper part of the base, the solution does not get reliable adhesion to the wall. The strength of the solution itself is higher than that of the base material, therefore, in the contact area, when the plaster mixture gains strength, peeling will occur. The primer, penetrating deep into the loose base, binds its individual particles, increasing strength.
2. A porous base is able to absorb moisture from the solution through its pores. And water is necessary for chemical reactions, due to which crystallization and strength gain of the plaster solution occurs. If the mixture loses water, strength is not gained. By clogging the pores, the primer prevents water from “leaking” from the solution.
3. Where moisture may appear on one of the contact sides, colonies of microorganisms can develop, leading to the appearance of stains and destruction of the finish. To prevent this, the base is impregnated with fungicidal compounds. These compounds are especially in demand for wood bases.
4. Prevention of efflorescence. There are special types of waterproofing compounds that form a dense film that does not allow water molecules to pass through.
5. Increasing adhesion in case of smooth bases. Quartz sand of a certain fraction is added to the impregnation. Sand grains allow the plaster mixture to cling to them.

Which primer is ideal to use before plastering walls is another question that beginners have.

Types of primers

The base material, mechanism of action, and component composition of primers vary.

Primer types are classified based on their chemical composition as follows:

• acrylic primers increase adhesion, are universal, quick-drying (not suitable for metals, because.. they provoke corrosion);
• alkyd (another name – phosphate) have an anti-corrosion effect, are suitable for wooden surfaces, the minus is a long (15 hours) drying;
• glyphthalic for metal structures of dry rooms;
• perchlorovinyl are universal in the base, for outdoor use;
• polyvinyl acetate (not popular, as they form an airtight film, the layer is short-lived) under PVA paint;
• phenolic (used as the first layer applied to metal or wood) for external use only;
• polystyrene external (for wood or plaster coating);
• epoxy highly adhesive, used for smooth bases (+ protection of metal surfaces from corrosion);
• shellac prevents resin release from wooden bases;
• mineral – for plastering concrete or brick walls.

By means of an action mechanism:

• universal;
• insulating – create a vapor-impermeable surface film, for example, hydrostop concentrates (white or blue);
• antifungal;
• adhesive to improve adhesion;
• deep penetrating used to impregnate loose, porous bases.

Though a white primer is typically used as a covering primer, the primer can be tinted.

Universal

Most frequently, universal water-soluble wall treatments with a silicate, acrylic, or silicone base are used during small repair work as a pre-plaster treatment.

These primers function in a multifaceted way.

• increase adhesion;
• have an antibiotic effect;
• strengthen the base.

When a single base contains a variety of materials and it is difficult to determine which to treat, primers are also used. When the base is made up of various materials with differing properties, a primer is utilized to balance and lessen the differences in the materials’ absorption characteristics. One such primer in KNAUF products is mittelgrund, which needs to be diluted with water.

Adhesive

They add roughness because of an abrasive or quartz filler, like needle quartz. They are made using organic materials or water (with alkyd resins). Bitcoin Kontakt is the most well-known. The principle of operation is comparable to that of double-sided tape.

Pre-applied to concrete, betonkontakt is an adhesive primer for gypsum plaster. The purpose of betonkontakt is to keep the concrete and gypsum from becoming wet after the gypsum composition is applied to the wall, ensuring that the solution does not slide.

Two filler types are included with Betonkontakt primer: quartz and sand. The image below demonstrates these compositions’ features and differences in a clear manner.

Concrete contact has an adhesive strength of 0.4 mPa, adds roughness, and zeroes down the base’s water absorption.

Because concrete contact has a high adhesion (for adhesives ≥ 0.5 mPa) and breaks the primer film when it hardens, using it as a substrate for tile adhesive or cement mortars will cause buckling and peeling of the coating.

Deep penetration

There is a lot of dust on the wall following grinding, stitching up seams and cracks, and removing the previous finish. The dust that has entered the pores cannot be cleaned out, but it can be partially removed. In the meantime, the plaster mortar base’s binders won’t be able to enter the pores and catch due to the dust. Consequently, the plaster layer’s adhesion will be unacceptablely low. In these situations, you require a product that has the ability to "bypass" dust and other small particles in order to deeply penetrate the base material and bind the loose structure. Impregnations with deep penetration are required.

Another name for these goods is tiefengrund. Deep penetration primers typically don’t really leave a film on the surface. Primer compositions easily penetrate deep into the wall material without forming a surface film because of the decreased dry matter content. Nearly all varieties of mineral bases are utilized with them (available for sale in ready-made or concentrated form).

The primer’s dry residue content is particularly significant. The primer needs to be more watery the deeper penetration that is needed. The dry residue, or polymer content, can be higher because larger pores make it easier for a thick composition to pass through them.

By diluting the primer concentrate, you can regulate the amount of dry residue. The viscous thick solution will not be able to penetrate deeply into the material if its pores and cracks are too small. Instead, it will form a thick layer that is waterproof and non-breathing on the surface.

For external work

Similar to plaster mixes, primers are classified according to their application area.

Products have been developed for external work that have:

• frost resistance;
• hydrophobicity;
• vapor permeability;
• weather resistance.

For brick and concrete facade finishing, for instance, a silicate primer works well.

For internal work

Some primers are not appropriate for indoor finishing. The product’s environmental friendliness is the primary requirement. Therefore, primers that release toxic substances upon drying are not used for interior finishing. Furthermore, you shouldn’t use a primer composition meant for dry settings in damp rooms.

Which primer is better for walls under plaster

A novice shouldn’t base their primer selection on the myth that using a well-known brand’s composition will solve all of their issues. It will be appropriate to use a composition with a suitable chemical formula for each unique base that corresponds to the task being solved by priming. As a result, you should consider the base material, plaster coating composition, and finish operating conditions when determining which primer is best.

Therefore, primers for external use are used for plastering facades, adhesive compounds with a mineral base are used as a primer for decorative plaster, and products that seal and penetrate pores are required for porous bases.

Dense, long-lasting bases shouldn’t be strengthened with primer because the unabsorbed material leaves a film that hinders adhesion.

Brick

In 90% of cases, a primer is required before plastering brick walls.

Brick can be used to create wall masonry:

• silicate (primers that provide high adhesion, for example, silicate for street use);
• ordinary.

Deep penetration primers are applied to old, crumbling brick bases in order to strengthen them. Additionally, this will fortify the masonry mortar that has started to fracture.

The following compositions work well for brick walls most of the time:

• water-soluble acrylic or silicone;
• mineral.

Brick is a porous substance that takes on water saturation. It is not necessary to prime the walls in order to apply plaster and a cement base. Before plastering, it is sufficient to moisten the brickwork; the solution won’t dry out. Extrude the seams and use a steel brush to lightly scratch the surface to improve adhesion.

How to prime concrete walls before plastering

Prior to plastering, almost any primer can be used to prepare concrete surfaces:

• acrylic (regular);
• acrylic structural (with quartz filler) – for decorative plaster;
• acrylate, alkyd, latex;
• concentrated primers (hydrostop).

The application of a deep-penetrating primer produces a surface that is nearly 100% waterproof. The ideal way to use gypsum is through concrete contact.

Wood

The "unpleasant" qualities of wood include its propensity to release resin, to swell, absorb moisture, burn easily, and become unstable when exposed to mold and fungus. Priming the walls is therefore a necessary step to protect the contact area when plastering wood. It is advisable to choose primers that have antiseptic and fire retardant additives.

Fibreboard, chipboard, plasterboard and OSB

While the bases of the first three materials are made of wood, the surface of plasterboard is made of cardboard. This establishes the qualities primers require. The surfaces must be given fungicidal and fire-retardant qualities, and they must be shielded from moisture without losing their vapor permeability. More adhesive is required for gypsum board and laminated fiberboard. Select universal, strengthening, and adhesive agents.

The factory paraffin coating must be removed before plastering OSB sheets with a primer.

It only takes one impregnation for plasterboard sheets. The uniformity of the sheets facilitates uniform penetration of the impregnation to the intended depth.

Foam block and aerated concrete

For finishing cellular concrete, a variety of well-known companies sell a unique primer. High porosity, the potential for biological colonies to form, and inadequately high surface layer strength are characteristics of aerated concrete and foam block plaster. Thus, a primer that lowers absorption capacity, strengthens, and is antiseptic is chosen for aerated concrete blocks.

Special wall primers have been developed for aerated concrete blocks, or cellular concrete in general. These primers create breathable surfaces once they dry. This is particularly crucial for surfaces that are outside.

Metal surfaces

Metal surfaces typically require rust prevention. Anti-corrosion agents are used for this, and they are applied to a surface that has been degreased (acetone is an option). It is advised to use specific agents that can convert rust. To improve adhesion, concrete contact is applied after they have dried. Reinforcement with a mesh is used, which is also primed, to improve the holding power of the plaster leveling layer (GF-019). Applying acrylic primer, which initiates the corrosion process, is not advised.

Insulation

Ceresit ST17 primer mixture is used to prime mineral wool. Mineral products with a silicate base are also appropriate. It makes sense to use adhesive compounds as a primer for penoplex. Deep penetration facade agents are applied to foam plastic.

Type of Primer	Best for
Deep Penetrating Primer	Porous surfaces like old plaster or brick
Adhesion Primer	Ensuring plaster sticks to smooth surfaces
Universal Primer	General use, suitable for most wall types
Mold-Resistant Primer	Damp or mold-prone areas
Alkali-Resistant Primer	Fresh concrete or lime-based surfaces

Plastering your walls after priming them is an essential step to guarantee a smooth, durable finish. By selecting the appropriate primer, you can reduce the likelihood of cracks and imperfections by creating a stable surface that improves plaster adhesion. It’s an investment in your work’s longevity and caliber.

Think about the kind of surface you’re working with when choosing a primer. Certain materials, like brick, drywall, or concrete, require different primers. To guarantee the best outcomes, pay close attention to the unique requirements of your walls.

Remember how crucial it is to mix and apply primer according to the manufacturer’s instructions. Applying the primer correctly will maximize its effectiveness and provide the best possible foundation for your plastering project.

Ultimately, you’ll save time and effort by taking the time to select the ideal primer and applying it correctly. Your walls will withstand the test of time, look better, and last longer.

A smooth, long-lasting finish requires priming walls before plastering, but selecting the appropriate primer can be challenging. This article makes the process easier by explaining the different types of primers that are available and how to choose the best one for the material and condition of your wall. Knowing your options will help guarantee that your plaster adheres correctly and maintains its perfect appearance for many years, regardless of whether you’re dealing with old paint, stains, or porous surfaces.

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