Introduction

FRP water tanks are widely used in municipal water supply, industrial liquid storage, and fire reserve water sources. Their service life and water quality safety directly depend on the quality of the anti-corrosion lining. Beijing Yuanhui FRP Co., Ltd. has observed through a decade of production and field construction that over 70% of lining failures originate from incorrect resin selection or process defects. This article breaks down key technical points from four dimensions: material selection, layup design, construction control, and quality inspection.

1. Resin and Curing System Matching

1.1 Corrosion Resistance Grades of Base Resins

The lining layer must use resins with excellent corrosion resistance. Recommended selections based on stored media:
- Drinking water: Food-grade isophthalic polyester resin (e.g., DSM P61-901) with Barcol hardness ≥40 and mass loss ≤0.5% after 72h water boiling.
- Industrial wastewater (pH 2-12): Bisphenol-A vinyl ester resin (e.g., Derakane 411-350) with flexural strength retention >85% after 30 days immersion in 10% H₂SO₄ and 20% NaOH.
- High-temperature media (≤80°C): Phenolic epoxy vinyl ester resin with HDT above 120°C.

1.2 Quantitative Control of Hardener and Accelerator

The curing system directly affects crosslink density and permeability resistance. Beijing Yuanhui's construction specification requires:
- MEKP hardener: 1.5%-2.0% by resin weight; upper limit at <15°C, lower to 1.2% at >30°C.
- Cobalt accelerator: 0.3%-0.5%; never mix directly with hardener. Use a gel timer (25°C, 25-35 min) to verify the ratio on site.

2. Reinforcement Materials and Layup Design

2.1 Functional Layering of Surface Veil and Chopped Strand Mat

The anti-corrosion lining typically uses a "sandwich" structure:
- Surface layer: 30g/m² polyester veil with ≥90% resin content (0.3-0.5mm thick) for chemical barrier.
- Intermediate layer: Alternating 450g/m² chopped strand mat with 70%-75% resin content for mechanical support.
- Structural layer: 800g/m² E-glass woven roving with 50%-55% resin content, integrated with the tank shell.

2.2 Overlap and Corner Treatment

Internal corners and weld areas are stress concentration points. Beijing Yuanhui requires:
- All corners chamfered to R≥20mm, with an extra layer of chopped strand mat as reinforcement.
- Overlap width ≥50mm, with adjacent layer joints staggered by ≥100mm. Infrared thermography must show a temperature difference ≤2°C at overlaps, or delamination is flagged.

3. Hand Lay-Up and Spray Process Essentials

3.1 Environmental Temperature and Humidity

Resin curing is sensitive to environment. Standards:
- Temperature 15-30°C, relative humidity ≤75%.
- If humidity exceeds 80%, a dehumidifier is mandatory; otherwise, water molecules react with isocyanate groups, causing whitening and a 30%-40% drop in flexural strength.

3.2 Bubble Elimination in Spray-Up

Spray-up is efficient but prone to air entrapment. Key points:
- Spray pressure 0.4-0.6 MPa, gun distance 300-400 mm.
- After each layer (~0.5mm), roll immediately with a de-bubbling roller at 1-2 m/min for 2-3 passes. Beijing Yuanhui's data shows pinhole rate drops from 15/m² (unrolled) to below 2/m² after rolling.

4. Quality Acceptance and Local Repair

4.1 Spark Test for Pinhole Detection

Acceptance follows ASTM D4787-13:
- Voltage: 5kV per mm of lining thickness (e.g., 10kV for 2mm).
- Criteria: ≤3 pinholes per m², no through-pinholes. Defective areas must be ground down to glass fiber, then repaired via cleaning → drying → resin patching → curing.

4.2 Barcol Hardness and Cure Verification

Use a portable Barcol impressor (GYZJ-934-1) on site:
- Lining Barcol hardness ≥40 (ASTM D2583).
- If <35, extend post-cure (≥72h at 25°C) or check hardener ratio. An acetone wipe test also helps: if the cotton ball sticks or softens the surface, curing is abnormal.

Conclusion

FRP water tank lining anti-corrosion is a systematic engineering task—from resin selection to construction environment, from layup design to inspection and repair. Beijing Yuanhui FRP Co., Ltd. recommends that project owners require complete construction records (temperature/humidity data, gel time test values, layer-by-layer photos) and engage a third party for spark testing. Only by integrating material science standards with field experience can a "zero-defect" lining achieve long-term corrosion protection.