How to Accurately Test the Coating Weight of Prepainted Steel Coil Substrate – A Step-by-Step Guide
📅 Published: August 25, 2026 | By Fanda Steel
📌 Summary: For buyers and quality control professionals, verifying the actual coating weight of prepainted steel coil substrates is essential to ensure you receive the specification you paid for. This guide provides a practical, laboratory-based method to test zinc or aluminum-zinc coating weight on GI and GL base materials.
Why Coating Weight Testing Matters
Prepainted steel coils (PPGI/PPGL) are widely used in roofing, wall cladding, and industrial applications. The substrate — typically hot-dipped galvanized steel (GI) or Galvalume steel (GL) — provides the fundamental corrosion resistance that determines the product's service life.
However, not all coils deliver the specified coating weight. Substitution with lower-grade materials (e.g., AZ30 instead of AZ150) is a common quality issue that only becomes apparent through testing.
This guide outlines a simple, reliable gravimetric method to determine the actual coating weight of the substrate — a test that can be performed in a basic laboratory setting.
Materials You Will Need
| Item | Purpose |
|---|---|
| Prepainted steel coil sample | Test material |
| Circular cutting die (80mm diameter) | To cut uniform samples |
| Analytical balance (0.001g accuracy) | For precise weighing |
| Concentrated sulfuric acid (H₂SO₄) | To strip the paint coating |
| Hydrochloric acid (HCl) | To strip the zinc/Al-Zn coating |
| Distilled water | For rinsing |
| Drying oven or hot air blower | To dry samples |
| Desiccator | To cool samples before weighing |
| Vernier calipers | To measure sample thickness (optional) |
Step-by-Step Testing Procedure
Phase 1: Sample Preparation
- Cut three circular samples from the prepainted coil using an 80mm diameter cutting die.
- Mark or label each sample for identification (e.g., Sample A, B, C).
- Measure the exact diameter of each sample to calculate the surface area.
Area (m²) = π × (Diameter in meters / 2)²
For an 80mm diameter sample: Area = 3.1416 × (0.080 / 2)² = 0.005027 m²
Total area = 0.005027
Phase 2: Paint Stripping (Remove the Organic Coating)
- Weigh each sample on the analytical balance and record the initial weight (W₁).
- Place the samples in a container filled with concentrated sulfuric acid (H₂SO₄).
- Allow to soak for several minutes — the acid will dissolve and remove the paint film completely. Observe until the coating is fully stripped.
- Remove the samples from the acid bath.
- Rinse thoroughly with distilled water to remove all acid residue.
- Wipe dry with a clean cloth, then dry completely using a drying oven or hot air blower.
- Cool the samples in a desiccator to room temperature.
- Weigh each sample again and record this weight (W₂).
W₁ – W₂ represents the weight of the paint coating (if needed for separate analysis).
The sample now consists of: steel substrate + zinc/Al-Zn coating.
Phase 3: Coating Stripping (Remove the Zinc/Al-Zn Layer)
- Place the stripped samples (from Phase 2) into a container filled with hydrochloric acid (HCl).
- Allow to soak until the zinc or aluminum-zinc coating is completely dissolved. Continue until bubbling stops and the surface appears as bare steel.
- Remove the samples from the acid bath.
- Rinse thoroughly with distilled water.
- Wipe dry with a clean cloth, then dry completely using a drying oven or hot air blower.
- Cool the samples in a desiccator to room temperature.
- Weigh each sample one final time and record this weight (W₃).
W₂ – W₃ represents the weight of the zinc/Al-Zn coating removed.
Phase 4: Calculation
Calculate the coating weight per square meter (g/m²) for each sample:
Example Calculation:
| Parameter | Value |
|---|---|
| Sample diameter | 80mm (0.080m) |
| Total area | 0.005027 m² |
| W₂ (after paint removal) | 16.9697g |
| W₃ (after coating removal) | 16.1720g |
| Weight difference (W₂ – W₃) | 0.7977g |
Coating Weight = 0.7977g / 0.005027 m² = 158.6 g/m²
This result corresponds to AZ150 (150 g/m²) coating specification.
What the Results Tell You
| Coating Weight Range | Corresponding Specification | Typical Application |
|---|---|---|
| 20–40 g/m² | AZ20 / Z20 | Short-term interior |
| 70–100 g/m² | AZ70 / Z100 | General exterior |
| 100–130 g/m² | AZ100 / Z120 | Standard roofing |
| 130–180 g/m² | AZ150 / Z180 | Coastal/premium |
| 180–220 g/m² | AZ200 / Z220 | High-corrosion areas |
If your specified AZ150 coil tests at 80 g/m², you have received a lower-grade product than ordered — a common quality issue that this test can identify.
Important Notes and Precautions
⚠️ Safety First:
- Concentrated sulfuric acid and hydrochloric acid are corrosive. Wear appropriate PPE (gloves, goggles, lab coat).
- Perform all acid handling in a fume hood or well-ventilated area.
- Neutralize acid waste properly before disposal.
📌 Accuracy Tips:
- Use an analytical balance with 0.001g precision for reliable results.
- Ensure samples are completely dry before each weighing — moisture will affect accuracy.
- Handle samples with tweezers to avoid contamination from fingerprints.
- Calculate the average of three samples for a representative result.
📋 Additional Considerations:
- This method measures the coating weight on both sides of the substrate (top and bottom).
- For prepainted coils, the paint layer is typically 15–25μm (PE) or 20–30μm (PVDF) — the paint weight can also be calculated from W₁ – W₂ if needed.
- For Galvalume (GL) substrates, the coating composition is 55% Al, 43.5% Zn, 1.5% Si — the same stripping method applies.
Why This Test Matters for Your Business
For buyers and quality assurance teams, this simple gravimetric test provides objective evidence of coating quality. It is:
- Quantifiable — Provides a specific, verifiable number in g/m²
- Reproducible — Can be repeated by any laboratory
- Defensible — Results can be documented and shared with suppliers
At Fanda Steel, we conduct this test on every batch of prepainted steel coils before shipment.
We believe in transparency and welcome third-party verification of our coating quality. When you specify AZ150, we deliver AZ150 — not a substitute.
📩 Contact Us for Coating Test Documentation or Technical Inquiries
Email: sales@fandasteel.com | WhatsApp: +86 185 5336 6695
