Oct 03, 2026 · 8 min read

PPGI Paint Coating Thickness: Primer, Topcoat, Backcoat

A single “20 microns total paint” figure is not enough to judge a prepainted galvanized steel (PPGI) specification. It tells you the combined dry film thickness (DFT) of all paint layers, but it does not tell you how that film is divided between primer, topcoat and backcoat, which resin systems are used, or how the coating will behave when the strip is roll formed, bent or exposed to a corrosive environment. As a practical starting point, many general-purpose exterior PPGI specifications split a 20-micron total into roughly 5 microns primer, 12–13 microns topcoat and 5 microns backcoat, but this is only a reference pattern. The correct split depends on the substrate, the zinc or zinc-alloy coating mass, the intended forming severity, the exposure environment and the specific paint chemistry. The buyer should specify each layer separately in the RFQ, not just the total.

PPGI paint coating thickness primer topcoat specification

Why total paint DFT is a weak specification

Total DFT is easy to measure and easy to compare, which is why it dominates quotations. But it hides the variables that actually determine performance.

Layer function differs. The primer provides adhesion to the metallic coating and corrosion inhibition. The topcoat provides colour, gloss, weather resistance and mechanical protection. The backcoat protects the reverse side and often serves as a forming lubricant.
Thin layers can fail independently. A 20-micron total made of 2 microns primer and 18 microns topcoat is not equivalent to 6 microns primer and 14 microns topcoat, even though the total is the same.
Resin chemistry matters as much as thickness. Polyester, silicon-modified polyester, polyurethane, PVDF and plastisol systems behave differently at the same DFT.
Curing and substrate condition are invisible in the number. A well-cured 18-micron system on clean, properly pretreated galvanized steel can outperform a poorly cured 25-micron system.

For a broader view of how paint systems sit on the metallic-coated substrate, see the PPGI coil product page and the GI coil page for the base substrate.

What each layer does

Primer

The primer is the adhesion and corrosion-inhibition layer. It bonds to the zinc or zinc-alloy surface and to the topcoat. Typical primers for PPGI are epoxy, polyester or polyurethane based, sometimes with anti-corrosive pigments. If the primer is too thin, adhesion and edge corrosion resistance can suffer. If it is too thick or incompatible with the topcoat, intercoat adhesion and flexibility can drop.

Topcoat

The topcoat carries the colour, gloss and most of the weather resistance. It also contributes to scratch and abrasion resistance. In exterior architectural use, topcoat thickness is usually the largest share of the total. In interior or less demanding applications, the topcoat may be thinner. Topcoat thickness alone does not guarantee colour retention or chalk resistance; the resin system and pigment selection matter.

Backcoat

The backcoat protects the reverse side during storage, transport and installation, and can act as a lubricant during roll forming. It is often thinner than the topcoat. Some specifications require a specific backcoat colour for identification or a specific backcoat for a particular forming process. A missing or underspecified backcoat is a common cause of reverse-side corrosion and coil-to-coil friction problems.

How primer, topcoat and backcoat interact

The three layers are a system, not a stack of independent numbers.

Adhesion: Primer-to-substrate and topcoat-to-primer adhesion depend on pretreatment, primer chemistry and cure. Increasing topcoat thickness will not fix poor primer adhesion.
Flexibility: Total DFT and the ratio of layers affect bend and T-bend performance. Very high total DFT can crack or craze on tight bends, especially at low temperatures.
Corrosion resistance: The metallic coating mass (zinc or zinc-aluminium) is the primary barrier. Paint layers slow the attack and protect cut edges. A thick topcoat over a thin primer may not improve salt-spray performance much.
Forming: Backcoat and primer lubricity influence friction, tool wear and surface damage during roll forming and bending.

This is why a specification should state the substrate, the metallic coating mass, the pretreatment type, the primer type and DFT, the topcoat type and DFT, the backcoat type and DFT, and the required mechanical and corrosion tests.

Practical specification table

The table below is general industry guidance for discussion, not a contract requirement. Confirm exact values with your supplier and your project specification.

Application context	Typical total DFT reference	Primer share	Topcoat share	Backcoat share
Interior panels, mild indoor exposure	15–20 microns	4–5 microns	8–12 microns	4–5 microns
General exterior roofing and cladding	20–25 microns	5–6 microns	12–15 microns	5–6 microns
Severe exterior or industrial environment	25–35 microns or higher	6–8 microns	18–25 microns	6–8 microns
Heavy forming or deep drawing	Depends on system	Flexible primer, exact DFT by trial	Flexible topcoat, exact DFT by trial	Lubricating backcoat

These ranges are indicative. A supplier may propose a different split based on their paint system and your forming and exposure requirements. Always ask for the layer-by-layer DFT, not just the total.

Common mistakes in PPGI paint specifications

Specifying only total DFT. This allows the supplier to shift thickness between layers in ways that may not suit your process.
Ignoring the metallic coating. Paint performance depends heavily on zinc or zinc-alloy coating mass. A thick paint system over a thin metallic coating may not meet corrosion expectations.
Using one specification for all environments. Interior, exterior, coastal and industrial exposures require different paint systems and thicknesses.
Overlooking backcoat. A thin or missing backcoat can cause reverse-side corrosion and forming problems.
Not stating test methods. Adhesion, bend, impact, salt spray and colour retention tests should be referenced with the exact method and acceptance criteria.
Assuming thickness equals durability. Resin chemistry, cure, pretreatment and substrate condition are equally important.
Quoting a standard without the edition. Standards such as those from ASTM, EN, ISO or GB are revised. Confirm the required edition and exact contract values.

RFQ checklist for PPGI paint systems

Use this checklist when requesting quotations. It helps you compare suppliers on the same basis.

Substrate: Base steel grade, thickness and tolerance.
Metallic coating: Type (zinc, zinc-aluminium, etc.) and coating mass per unit area, with test method.
Pretreatment: Type and process control requirements.
Primer: Resin type, DFT range, colour if relevant.
Topcoat: Resin type, DFT range, colour, gloss range, and whether the colour is a standard or a custom match.
Backcoat: Resin type, DFT range, colour and function (protective, lubricating or both).
Total DFT: Minimum, nominal and maximum, with measurement method.
Mechanical properties: T-bend, adhesion, impact and pencil hardness requirements.
Corrosion tests: Salt spray or cyclic test method and duration, with acceptance criteria.
Weathering tests: QUV, xenon-arc or outdoor exposure method and acceptance criteria, if applicable.
Forming: Intended roll forming, bending or profiling operations, and any known tight radii.
Handling and storage: Coil wrapping, edge protection and storage conditions.
Documentation: Mill test certificate, coating mass report, DFT report and test reports.
Commercial: Quantity, coil size, tolerance, packing, delivery terms and validity of the quotation.

If you are comparing offers, ask each supplier to state the layer-by-layer DFT and the test methods they will use. This makes the comparison meaningful.

FAQs

Is 20 microns total paint enough for PPGI?

It can be adequate for some general exterior applications, but it depends on the environment, the metallic coating mass, the resin system and the forming process. For severe environments or aggressive forming, a higher total DFT or a different paint chemistry may be needed. Always confirm the layer-by-layer breakdown and the test requirements.

How should I split primer, topcoat and backcoat?

A common reference for general exterior use is about 5–6 microns primer, 12–15 microns topcoat and 5–6 microns backcoat within a 20–25 micron total. This is not a universal rule. Your supplier should propose a split based on your substrate, environment and forming requirements, and you should verify it with appropriate tests.

Does thicker paint always mean better corrosion resistance?

No. Corrosion resistance depends heavily on the metallic coating mass, pretreatment quality, paint adhesion and the environment. Increasing paint thickness beyond the system’s design can reduce flexibility and cause cracking on forming. The right combination matters more than the total number.

What standards apply to PPGI paint thickness?

Different markets use different standards, such as ASTM, EN, ISO or GB. Each standard may specify measurement methods and acceptance criteria. Confirm the required edition and the exact contract values, because standards are revised and project specifications can add requirements.

Can I specify the same paint system for interior and exterior use?

It is possible, but it may not be cost-effective. Interior applications often need less weather resistance and can use a thinner or different topcoat. Exterior applications usually need a topcoat with better UV and weather resistance. Discuss the exposure conditions with your supplier.

How does price relate to paint specification?

Paint system cost is driven by resin type, pigment selection, layer thicknesses, substrate metallic coating mass, pretreatment and testing requirements. A higher total DFT or a more durable resin system generally increases cost. Because raw material and energy costs change, request a dated quotation and confirm what is included in the price.

Next step

If you are preparing an RFQ or comparing supplier offers, send your application details, forming requirements and exposure conditions. ChinaCNU Steel can review your specification and respond with a layer-by-layer coating proposal for discussion. Visit the contact page to submit your requirements and request a dated quotation.