Choosing and Applying Waterproof Coatings — The Critical Role of Primer, Mid-coat, and Topcoat
In construction waterproofing projects, coating systems typically include a primer, mid-coat, and topcoat. Among these, polyurethane (PU) coatings play a central role due to their tensile strength and elongation properties, which directly impact the durability, adaptability, and protective performance of the waterproof layer. This article explores the purpose of each layer and provides performance comparisons to help select the most suitable waterproofing solution.
🧱 Purpose of Waterproof Primer
The primer enhances adhesion and durability while improving construction quality. Its functions include:
- Improved adhesion: Fills micro-pores in the substrate, allowing the waterproof layer to bond firmly to the structure.
- Water seepage prevention: Penetrates the substrate to form a protective layer, reducing moisture infiltration and minimizing mold or efflorescence risks.
- Enhanced durability: Stabilizes the substrate, preventing aging or peeling, and extends the waterproof layer’s lifespan.
- Better application results: Smooths the surface, reduces topcoat absorption, and improves overall coating quality.
Since primer significantly affects the adhesion of the mid-coat, it’s recommended to coordinate with the mid-coat supplier when purchasing.
🧱 Purpose of Waterproof Mid-coat
The mid-coat is the core layer ensuring waterproof effectiveness. Its main functions include:
- Increased layer thickness: Fills gaps between primer and topcoat and can be reinforced with polyester or fiberglass for added protection.
- Improved weather resistance: Withstands temperature changes, UV exposure, and chemical erosion, extending service life.
- Better application quality: Creates a more uniform waterproof layer, reducing cracks and seepage risks.
- Impact resistance: Even if the topcoat is damaged, the mid-coat maintains waterproof integrity.
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🌍 Tensile Strength of Mid-coat: A Key Factor in Seismic Zones
In earthquake-prone areas, waterproof materials with excellent elongation after film formation ensure the layer doesn’t crack due to minor structural shifts. High tensile mid-coats can absorb external stress, preventing stress concentration and cracking, and resist moisture infiltration caused by thermal expansion and contraction. Ideal applications include:
- Rooftop waterproofing
- Basement waterproofing
- Floor slab waterproofing
🧪 Recommended Mid-coat Products
| Characteristics |
Product Code |
| Complies with CNS 8644 — over 95% of properties meet the highest standard. |
CPU-49-WP101I-A/B |
| Complies with CNS 8644 — over 85% of properties meet the highest standard. |
CPU-184-UT50B-A/B |
| Complies with CNS 8644 — over 65% of properties meet the highest standard. |
CPU-49-MF-14H-A/B |
| Easy-to-use single component / Near high standard |
CPU-49-MF12F |
| Easy-to-use single component / High elongation |
CPU-49-WP-19J |
| High transparency, weather & corrosion resistance / Can be used as topcoat |
CPU-13-C-92AN-A/B, CPU-13-C-92FN-A/B |
🧪 Midcoat Comparison (Single Component)
| Brand |
TPUCO |
Brand S |
Brand T |
Brand M |
| Product |
CPU-49-WP-19J |
6xx |
2xx |
2xx |
| Tensile Strength (N/mm²) |
35+ |
30–45 |
35+ |
35+ |
| Elongation (%) |
600+ |
200–450 |
600+ |
600–900 |
🎨 Purpose of Waterproof Topcoat
The topcoat is the outermost layer of the waterproof system, providing:
- Extra protection: Enhances weather resistance against UV rays, rain, and pollutants.
- Aesthetic appeal: Improves surface smoothness and uniformity to meet architectural design needs.
- Durability: Reduces wear and tear, extending service life.
- Anti-stain and anti-mold: High-quality topcoats prevent dampness and mold, keeping buildings in good condition.
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💪 Tensile Strength of Topcoat: Preventing Cracks and Enhancing Durability
- Adapts to substrate movement: High tensile strength prevents cracking from structural shifts or temperature changes.
- Crack prevention: Withstands external stress to avoid fissures.
- Improved abrasion resistance: Ensures long-term waterproofing under various environmental conditions.
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🧪 Topcoat Comparison (Single Component)
| Brand |
TPUCO |
Brand S |
Brand T |
Brand M |
| Product |
CPU-49-TC-10B |
7xx |
3xx |
4xx |
| Tensile Strength (N/mm²) |
200+ |
200+ |
35+ |
35+ |
| Elongation (%) |
130+ |
200+ |
100+ |
100+ |
☀️ For Long-Term Sun Exposure: Non-Yellowing White PU Topcoat
For areas exposed to sunlight (e.g., rooftops), white PU topcoats are recommended due to:
- UV and weather resistance: Contains UV-blocking agents to prevent yellowing and aging.
- High reflectivity: Reflects sunlight, reduces surface temperature, and improves energy efficiency.
- Long-lasting appearance: Maintains cleanliness and resists discoloration.
🛡️ For High Budgets and Extreme Performance Needs: Polyurea
Polyurea coatings with tensile strength over 300+ N/mm² offer superior durability and abrasion resistance, ideal for:
- Outdoor structures
- Industrial equipment
- Flooring systems
🏗️ Key Considerations When Choosing Waterproof Materials
Waterproof materials must meet theoretical standards and pass lab tests—but proven success in real-world applications is even more important. Brands with extensive track records are generally more reliable.
Internationally renowned waterproofing brands include:
- Sika
- Tremco
- Maris Saint-Gobain
These companies specialize in waterproofing, sealing, and flooring solutions, widely used in airports, government buildings, and commercial complexes.
Taiwan’s TPUCO has developed high-quality waterproof coatings over many years, offering performance comparable to international brands at a competitive price—making it a top choice for commercial buildings and high-rises.
Link to TPUCO products & PU Knowledge:
Fast drying polyurethane coating:
https://www.taiwanpu.com/en/product/Fast-drying-polyurethane-coating
Q27:Key Steps for High-Quality Waterproofing
https://www.taiwanpu.com/en/about/Key-Steps-for-High-Quality-Waterproofing
Q28:How to Choose High-Quality Waterproofing Materials
https://www.taiwanpu.com/en/about/How-to-Choose-High-Quality-Waterproofing-Materials
Q29:CNS 8644: Safeguarding Waterproofing Quality
https://www.taiwanpu.com/en/about/CNS-8644-Safeguarding-Waterproofing-Quality
Q30:CNS 8644 Roofing & Balcony Waterproofing Guide
https://www.taiwanpu.com/en/about/CNS-8644-Roofing-and-Balcony-Waterproofing-Guide
Q31:TPUCO’s CNS 8644-Compliant Waterproof Coatings
https://www.taiwanpu.com/en/about/TPUCO-CNS-8644-Compliant-Waterproof-Coatings
Q32:Team or DIY? Choosing the Right Waterproofing Plan
https://www.taiwanpu.com/en/about/Team-or-DIY-Choosing-the-Right-Waterproofing-Plan
Q33: Why Waterproofing Needs Primer, Mid & Topcoat?
https://www.taiwanpu.com/en/about/Why-Waterproofing-Needs-Primer-Mid-Topcoat
Q34:Waterproofing Project Case Study & Performance Showcase
https://www.taiwanpu.com/en/about/Waterproofing-Project-Case-Study-and-Performance-Showcase
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Note: This content integrates international academic literature, internal lab data, and real-world usage. Results may vary depending on proprietary formulas and application methods.
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📧 Email:
pu@taiwanpu.com