I. Scenarios Where Waterproof Primer "Must Be Used" (Omission Will Directly Cause Waterproofing Failure)
Porous/loose substrate scenarios
When the substrate is a porous material such as concrete, cement mortar, aerated brick, or gypsum board, primer is a "must-have":
Such substrates have a large number of capillary pores on their surfaces. If waterproof coating is applied directly, the resin and emulsion in the coating will be quickly absorbed by the substrate, leading to "excessive dry shrinkage, insufficient thickness" of the coating, and even "chalking and cracking";
The interior of the substrate may retain moisture. The primer can fill the pores through permeability, forming a "sealing layer" to block the upward penetration of moisture, and prevent the waterproof layer from "hollowing and peeling" due to moisture expansion in the later stage (e.g., concrete surfaces in basements and bathrooms must be primed first).
Smooth/low-adhesion substrate scenarios
When the substrate is a smooth or low-polarity material such as metal (steel structure roofs), ceramic tiles/stone (old substrate renovation), glass, or plastic, the primer acts as a "bonding bridge":
The surface of such substrates is smooth, making it difficult for waterproof coatings to adhere directly (e.g., applying polyurethane coating on a metal roof will cause peeling due to insufficient adhesion if constructed directly);
Specialized primers (such as "epoxy zinc-rich primer" for metals and "interface treatment primer" for ceramic tiles) can improve the surface polarity and roughness of the substrate through chemical modification or physical roughening (e.g., containing quartz sand), allowing the waterproof coating to bond firmly with the substrate and preventing the waterproof layer from "slipping and peeling".
High-humidity/long-term water-exposed environments
For areas that are in long-term contact with water or have high humidity, such as bathrooms, kitchens, swimming pools, roofs, and underground garages, the "moisture-proof and sealing effect" of the primer is irreplaceable:
Even if the substrate seems dry, there may be trace moisture inside, which will accumulate in long-term water exposure;
The primer can seal the substrate pores in advance, preventing water from "flowing between the waterproof layer and the substrate" and avoiding damage to the waterproof layer due to "water pressure + moisture expansion" (e.g., if no primer is applied to the roof waterproof layer, rainwater may seep into the substrate through the capillary pores of the substrate, causing mildew and leakage inside the roof).
Supporting requirements for specific types of waterproof coatings
The formula design of some waterproof coatings includes primer as a "mandatory supporting material"; omission will violate the product construction specifications:
For example, polyurethane waterproof coatings, asphalt-based waterproof coatings, and solvent-based waterproof coatings: These coatings have poor compatibility with inorganic substrates (such as concrete) and must transition through primers (e.g., polyurethane-specific base coats, asphalt-matched primers) to achieve effective bonding;
Manufacturers usually clearly state in the product manual that "a dedicated supporting primer must be used". If construction is not carried out as required, leakage in the later stage may not be covered by the warranty.
II. Scenarios Where Waterproof Primer "May Be Omitted at Discretion" (But Priority Use Is Still Recommended)
Primer can only be omitted under special conditions where the substrate is extremely good, the coating is compatible, and the environment is dry-but certain risks must be borne:
The substrate is a "completely dense, dry, and smooth surface"
Example: New, polished high-density ceramic tiles (no cracks or pores), dry epoxy floor surfaces (already sealed), and the applied coating is a "high-adhesion coating" (e.g., JS cement-based waterproof coating, which itself has strong compatibility with inorganic materials). In this case, the primer can be omitted-but the substrate surface must be cleaned with water in advance to ensure no oil or dust, and thin, multiple coats must be applied during painting to avoid coating sagging.
Dry "non-critical waterproof areas"
Example: Dry balconies (only for occasional rain protection), walls of dry indoor storage rooms (only for condensation prevention), where the substrate is dry cement mortar (with few pores) and the coating is a "high-permeability cement-based coating" (which itself has a certain sealing effect). In this case, the primer can be omitted-but note that if the environmental humidity increases later (e.g., the balcony is converted into a laundry area), the waterproof layer still risks cracking and leakage.
III. Core Recommendation: Prioritize Using Waterproof Primer Instead of "Omitting It When Possible"
Even in the few scenarios where "omission is possible", the industry still recommends "prioritizing primer use" for the following reasons:
Cost perspective: The unit price of primer is much lower than that of waterproof coating. Using primer can reduce the "invalid loss" of waterproof coating absorbed by the substrate (usually saving 15%-30% of coating consumption), which actually reduces the overall material cost;
Effect perspective: Primer can repair minor defects of the substrate (such as fine cracks and pinholes), providing a "flat and solid surface" for the waterproof coating. This allows the waterproof layer to form a continuous, non-breaking film, significantly reducing the risk of leakage in the later stage;
Service life perspective: A waterproof layer with a primer has higher bonding strength with the substrate, stronger tensile resistance, and better aging resistance (e.g., for a roof waterproof layer, the primer can reduce "interlayer peeling" caused by temperature changes), extending the service life by 3-5 years.
Conclusion
The "necessity" of waterproof primer essentially lies in "solving the 'compatibility issue' between the substrate and the waterproof coating". If the substrate and coating can directly achieve "strong bonding, no absorption, and no moisture interference", omission is possible; otherwise, it must be used. However, from the perspective of project reliability, unless there are clear product instructions and professional engineers confirm that "primer is not needed", all waterproofing projects should prioritize the use of a dedicated primer that matches the substrate and coating. This avoids greater losses caused by waterproofing failure and rework due to "saving small costs".
