If you are comparing quotes from PPGI suppliers and seeing some specify dry film thickness (DFT) while others specify total build thickness (TBT), the short answer is: you need to specify both, but DFT is the number that governs corrosion and durability performance. TBT is a useful process control check, but DFT per layer tells you whether the primer, topcoat, and any backing coat actually meet your project’s environmental demands. This article explains the difference, why it matters for your specification, and how to avoid costly mismatches between supplier quotes and field performance.
What Is DFT and What Is TBT in PPGI Coils?
PPGI (pre-painted galvanized iron) coil is a continuous coated steel product. The paint system is applied in multiple layers over a metallic coating, usually hot-dip galvanized (GI) or Galvalume. When you read a coating specification, you will encounter two thickness terms:
- DFT (Dry Film Thickness): The thickness of a single, dried paint layer. For a standard PPGI system, you will typically see a primer DFT and a topcoat DFT. Some specifications also include a backing coat DFT on the reverse side.
- TBT (Total Build Thickness): The sum of all dry paint layers on one side, excluding the metallic coating. For example, primer DFT (5 microns) + topcoat DFT (20 microns) = TBT of 25 microns.
The confusion arises because some suppliers quote only TBT, which can hide an imbalanced system. A TBT of 25 microns could mean 10 microns of primer and 15 microns of topcoat, or 2 microns of primer and 23 microns of topcoat. The first system will likely perform better in corrosive environments because the primer provides adhesion and anti-corrosion pigments. The second system may crack or delaminate prematurely.
Why DFT Matters More Than TBT for Durability
Corrosion protection in PPGI works through a layered defense. The zinc or aluminum-zinc metallic coating provides sacrificial protection at cut edges and scratches. The paint system provides a barrier against moisture, oxygen, and UV radiation. Within that paint system, the primer is the adhesion bridge and the primary corrosion-inhibiting layer. The topcoat provides color retention, UV resistance, and a dense barrier.
If your supplier only guarantees TBT, they can legally (in a contractual sense) shift thickness from the primer to the topcoat to reduce cost. This is not necessarily deceptive, but it is a risk. A thicker topcoat with a thinner primer may look fine initially but fail earlier in salt-laden or industrial environments. Therefore, your specification should always state the minimum DFT for each individual layer, not just the total.
How to Read a PPGI Coating Specification Correctly
When you receive a technical datasheet from a supplier, look for the following lines:
- Primer type and DFT: Usually 5–10 microns for polyester (PE) systems, sometimes higher for PVDF or SMP.
- Topcoat type and DFT: Typically 15–25 microns for PE, 20–25 microns for SMP, and 20–25 microns for PVDF.
- Backing coat (reverse side): Often 5–10 microns of a thin epoxy or polyester, primarily for protection during transport and storage.
- TBT (if stated): The arithmetic sum of the above layers on the top side.
Remember that DFT is measured on the dry paint film, not the wet film. All reputable suppliers use a magnetic or eddy-current gauge to verify DFT on the finished coil. However, there is a tolerance. Industry practice often allows a variation of ±10% on the specified DFT, but you must confirm this tolerance with your supplier and write it into your contract.
Typical DFT Ranges for Common Resin Systems
The resin system (the polymer binder) dictates the recommended DFT range. If you apply a resin below its minimum recommended DFT, the film may be porous. If you apply it above the maximum, you risk solvent entrapment and poor cure. Here is a general guide:
| Resin System | Typical Primer DFT (microns) | Typical Topcoat DFT (microns) | Typical TBT (microns) |
|---|---|---|---|
| Polyester (PE) | 5–8 | 15–20 | 20–28 |
| Silicone Modified Polyester (SMP) | 5–8 | 18–22 | 23–30 |
| Polyvinylidene Fluoride (PVDF) | 5–8 | 20–25 | 25–33 |
| High Durability Polyester (HDP) | 5–8 | 20–25 | 25–33 |
These values are general industry guidance, not a universal standard. Your project may require a different balance. Always confirm the exact resin system and DFT values with your supplier and, if possible, with an independent coating consultant for critical projects.
Practical Decision Table: DFT vs. TBT in Your Purchase Order
Use this table to decide what to specify and how to verify it when comparing supplier quotes.
| Scenario | What to Specify | What to Verify | Why It Matters |
|---|---|---|---|
| Coastal or high-humidity environment | Primer DFT ≥ 8 microns; Topcoat DFT ≥ 20 microns; TBT ≥ 28 microns | Confirm primer type (e.g., epoxy or polyurethane-modified) and metallic coating weight (e.g., Z275 or AZ150) | Thin primer fails first in salt exposure; edge corrosion accelerates. |
| Industrial or chemical atmosphere | Primer DFT ≥ 8 microns; Topcoat DFT ≥ 22 microns; TBT ≥ 30 microns | Ask for the specific anti-corrosive pigments in the primer | Chemical exposure requires a dense barrier; TBT alone does not guarantee barrier quality. |
| Roofing sheet for standard rural or urban use | Primer DFT ≥ 5 microns; Topcoat DFT ≥ 15 microns; TBT ≥ 20 microns | Check the backing coat DFT (should be ≥ 5 microns) to prevent condensation corrosion | Lower cost systems are acceptable but only for mild environments. |
| Architectural or high UV exposure | Topcoat DFT ≥ 20 microns; use PVDF or HDP resin | Confirm the topcoat is a true PVDF (≥70% PVDF resin by volume) or HDP | UV resistance is a function of resin and topcoat thickness, not just total build. |
| Supplier quotes only TBT | Request a layer-by-layer DFT breakdown in writing | Reject the quote if they cannot provide minimum DFT per layer | You cannot assess corrosion risk from TBT alone. |
Common Mistakes When Specifying PPGI Paint Thickness
Even experienced procurement managers make these errors. Avoid them to protect your project.
Mistake 1: Specifying Only TBT in the Purchase Order
This is the most common error. It allows the supplier to reduce primer thickness. Always write the DFT for each layer in your PO and technical annex.
Mistake 2: Confusing Metallic Coating Weight with Paint Thickness
The zinc coating (e.g., Z275) is measured in grams per square meter (gsm), not microns of paint. A high zinc coating does not compensate for a thin primer. Both are independent specifications. For detailed guidance on the metallic layer, see our GI coil technical overview.
Mistake 3: Ignoring the Backing Coat
The reverse side of the coil often gets a thin backing coat. If your application is a roof sheet with a visible underside in a humid environment, a backing coat DFT of at least 5 microns is necessary to prevent condensation-driven corrosion.
Mistake 4: Assuming All Suppliers Measure DFT the Same Way
Ask how the supplier measures DFT. The correct method is on the finished coil after curing, using a calibrated gauge. Some suppliers may calculate theoretical DFT from wet film thickness or from paint consumption, which can be inaccurate.
Mistake 5: Not Specifying the Test Standard and Tolerances
If you reference a standard like ASTM D7095 or ISO 2808, confirm the exact edition and the acceptable deviation with your supplier. Otherwise, a dispute can arise over a 2-micron difference.
RFQ Checklist: What to Ask Your PPGI Supplier
When you request a quote, include the following points in writing. This reduces the risk of ambiguity and gives you a basis for inspection.
- Resin system: PE, SMP, PVDF, or HDP. State the brand or generic chemistry.
- Primer DFT: Minimum value in microns, measured on the dry film.
- Topcoat DFT: Minimum value in microns.
- Backing coat DFT: Minimum value in microns (if required).
- TBT: State the calculated minimum total, but only as a secondary check.
- Metallic coating: Specify the coating weight (e.g., Z275) and base steel thickness separately.
- Test method: Ask which standard (e.g., ISO 2808) will be used for verification and what tolerance applies.
- Inspection reports: Request a copy of the coil coating line’s DFT measurement report for each batch.
- Sample retention: Ask if the supplier retains a cut sample from each coil for future verification.
Price Implications of DFT and TBT
Paint thickness is a major cost driver. More paint means more raw material and slower line speed. A thicker primer and topcoat will increase the price per ton of PPGI. However, the cost difference between a 20-micron TBT and a 28-micron TBT is usually small compared to the cost of premature failure and re-roofing. You should request a dated quote for each DFT specification you are considering. Prices for steel and paint resins fluctuate, so do not rely on current market chatter; ask for a formal quotation with a validity date. For a full list of PPGI options, review our PPGI coil product range.
Service Life: What DFT Can and Cannot Guarantee
You may see marketing claims about “25-year warranty” or “30-year durability.” Be cautious. No supplier can honestly guarantee a fixed service life for a paint system because actual performance depends on several variables outside the coil producer’s control:
- Substrate quality: The base steel and metallic coating weight.
- Paint system: Resin type, primer quality, and DFT balance.
- Environment: UV intensity, salt, humidity, chemical pollution, and temperature cycles.
- Forming and installation: Sharp bends, scratches, and cut edges that expose the metallic coating.
- Maintenance: Washing, debris removal, and repair of scratches.
A higher DFT generally improves durability, but it is not a guarantee. Use DFT as a specification control, not as a promise of longevity. If a supplier offers a warranty, ask for the exact terms, exclusions, and the inspection procedure in writing. Do not accept a verbal assurance.
Frequently Asked Questions
Q1: Can I use TBT alone to compare two PPGI suppliers?
No. TBT alone can hide a thin primer. Always ask for the DFT of each layer. If one supplier quotes TBT 25 microns and another quotes TBT 25 microns, the first may have 3 microns of primer and 22 microns of topcoat, while the second has 8 microns of primer and 17 microns of topcoat. The second will typically be more corrosion-resistant.
Q2: What is the minimum DFT for a primer on a coastal roof?
For a coastal environment, a primer DFT of at least 8 microns is commonly recommended, but this is not a universal rule. Confirm the primer chemistry and the metallic coating weight with your supplier. A higher primer DFT may be necessary if the environment is particularly aggressive.
Q3: How is DFT measured on a PPGI coil?
DFT is measured using a magnetic induction gauge (for steel substrate) or an eddy-current gauge (for non-ferrous metallic coatings). The gauge is placed on the cured paint film. Measurements are taken at several points across the coil width and along the length. Ask your supplier for the measurement protocol and the number of readings per coil.
Q4: Does a thicker topcoat always mean better UV resistance?
Not necessarily. UV resistance is primarily a function of the resin system (e.g., PVDF vs. polyester) and the quality of the pigments. A thicker polyester topcoat will not outperform a properly specified PVDF topcoat of lower thickness. For high UV exposure, choose the correct resin, then specify the minimum topcoat DFT recommended for that resin.
Q5: What is the difference between TBT and total coating thickness including the zinc layer?
TBT refers only to the paint layers. The total coating thickness on the top side includes the metallic coating (zinc or aluminum-zinc) plus the paint layers. For example, a Z275 coating (approximately 20 microns per side) plus a 25-micron paint TBT gives a total of about 45 microns on the top side. However, the metallic coating and paint are specified separately because they perform different functions.
Q6: Can I request a specific DFT value that is higher than the supplier’s standard?
Yes, but this may affect price and lead time. Most coil coaters can adjust the paint application within a practical range. You should request the exact DFT values in your RFQ and ask the supplier to confirm their maximum achievable thickness for the chosen resin system without compromising cure speed.
Final Recommendation for Your Coating Specification
To ensure your PPGI coil meets your project’s corrosion and durability requirements, write your specification with minimum DFT per layer as the primary control, and use TBT only as a secondary check. Include the resin system, metallic coating weight, and backing coat requirements. Confirm the test standard and tolerance with your supplier before ordering. This approach protects you from ambiguous quotes and gives you a clear basis for quality inspection upon delivery.
If you are preparing a specification and need a technical review of your DFT requirements, contact our team with your project environment details. We can help you translate your performance needs into a clear, verifiable coating specification.
Ready to specify your next PPGI order with confidence? Send us your RFQ with your required DFT values, resin system, and metallic coating weight. Our team will confirm the technical feasibility and provide a dated quotation. Contact ChinaCNU Steel here to discuss your project.
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