1. Seal Substrate Pores and Block Moisture/Water Penetration
Building substrates (especially concrete and cement mortar substrates) have a large number of tiny pores and capillary channels on their surfaces, and may retain moisture or dampness inside. If waterproof coating is applied directly:
The active ingredients in the coating will be "absorbed" by the substrate pores, resulting in insufficient coating thickness, slow drying, and even "chalking";
The moisture inside the substrate will expand when heated in the later stage, lifting the waterproof layer and causing hollowing, cracking, and peeling, which directly damages the waterproof effect.
Function of the primer: Through its permeability, it fills the tiny pores on the substrate surface, forming a dense "sealing film" to block the upward penetration of moisture inside the substrate. At the same time, it prevents the subsequent waterproof coating from being excessively absorbed by the substrate, ensuring that the coating can form a uniform and complete waterproof film on the surface.
2. Enhance the Adhesion Between the Waterproof Layer and the Substrate
Waterproof coatings (such as polyurethane, JS cement-based, asphalt-based, etc.) have poor "material compatibility" with substrates (inorganic materials like concrete, metal, wood, etc.), and direct bonding is prone to "delamination".
Function of the primer:
Two-way bonding bridge: The primer can not only form chemical or physical adsorption with the substrate (e.g., silicate components in concrete, oxide layers on metal surfaces) but also form a firm bond with the polymer components of the subsequent waterproof coating, preventing the waterproof layer from peeling off due to insufficient adhesion;
Improve the surface polarity of the substrate: For example, the surface of a metal substrate is smooth and low in polarity, making it difficult for waterproof coatings to adhere. A special metal primer can improve surface roughness and polarity through chemical modification, significantly enhancing the bonding effect.
3. Level and Repair Minor Defects of the Substrate
The substrate surface may have fine cracks (such as concrete drying shrinkage cracks), pinholes, or protruding particles. If not treated:
The waterproof coating will easily form "weak points" at the defects, which are prone to leakage under water pressure or external force in the later stage;
Protruding particles will cause uneven coating application and insufficient local thickness.
Function of the primer:
Primer with good fluidity can fill tiny cracks and pinholes, forming a smooth transition layer;
It has a certain "encapsulation and solidification" effect on floating sand and loose particles on the substrate surface, reducing the impact of impurities during subsequent construction and providing a smooth and solid construction base for the waterproof coating.
4. Save the Usage of Waterproof Coating and Reduce Costs
If the substrate has high porosity, a large amount of coating will seep into the pores (called "absorption loss") when waterproof coating is applied directly. This requires multiple coats to reach the designed thickness, increasing material costs.
Function of the primer: By sealing the substrate pores, it reduces the "invalid penetration" of the waterproof coating, allowing the subsequent coating to be mainly used to form a continuous waterproof film on the surface. Generally, it can save 15%-30% of the waterproof coating usage, reduce the number of coats, and improve construction efficiency.
5. Adapt to Different Substrates and Improve the Compatibility of the Waterproof System
Different building substrates (such as concrete, ceramic tiles, metal, wood, old waterproof layers) have significant differences in material properties, so primers need to be selected according to specific conditions to avoid "material conflicts":
For example, when applying a new polyurethane coating on an old asphalt waterproof layer, direct construction is prone to "swelling reaction". Using a special isolation primer can block the chemical interaction between the two and ensure compatibility;
Wood substrates are prone to moisture absorption and deformation. A special wood primer can not only seal pores but also play a certain role in anti-corrosion and moisture-proofing, extending the service life of the substrate.
Common Primer Types and Their Core Functions for Different Substrates (Reference)
| Substrate Type | Common Primer Types | Key Focus of Core Functions |
|---|---|---|
| Concrete/Cement Mortar | Cement-based Permeable Crystalline Primer, Acrylic Primer | Seal pores, enhance adhesion, block moisture |
| Metal (Steel Structure) | Epoxy Zinc-rich Primer, Phosphating Primer | Rust prevention, improve surface polarity, enhance adhesion |
| Wood | Water-based Wood Sealing Primer, Alkyd Primer | Seal wood grain, anti-corrosion and moisture-proof, prevent coating penetration |
| Old Waterproof Layer (Asphalt/Roll) | Asphalt-based Isolation Primer, Neoprene Primer | Isolate new and old materials, avoid swelling, enhance adhesion |
| Ceramic Tiles/Stone | Interface Treatment Primer (with Quartz Sand) | Improve surface roughness, prevent coating slipping |
In summary, the primer is not an "optional auxiliary material" but a crucial linking component in the building waterproof system that "connects the substrate and the waterproof layer". Its quality and whether it is suitable for the substrate directly determine the adhesion strength, durability, and waterproof effect of the waterproof layer. Especially in areas prone to long-term water erosion such as basements, roofs, and bathrooms, the role of the primer is even more important.
