How to Inspect Coating Thickness Correctly

For fasteners and metal components used in construction, solar mounting, infrastructure and machinery, coating thickness is one of the most critical quality parameters. A coating that is too thin may lead to premature corrosion and failure. A coating that is too thick can cause thread interference, flaking, or poor appearance.

Importers, distributors and project buyers need simple, reliable methods to check coating thickness on incoming shipments. At the same time, factories must follow recognised standards and use suitable instruments so that results are consistent from batch to batch.

This guide explains the basics of coating thickness, the main measurement methods, and practical inspection steps you can adopt in your own QC procedures when sourcing bolts, nuts, screws, concrete fasteners and other metal parts.

1. Why coating thickness matters

A coating system is designed to give a certain level of corrosion protection and service life. That performance is usually based on a specified dry film thickness (DFT). If the DFT is below the specified minimum, the coating may not protect the steel or other substrate for the expected time.FROSIO

Typical problems caused by incorrect coating thickness include:

  • Under-thickness
    • Earlier rust and pitting on steel components
    • Failure to meet project specifications and inspection requirements
    • More frequent repainting or replacement
  • Over-thickness
    • Thread interference on bolts, nuts and screws
    • Cracking or delamination of brittle coatings such as zinc flake
    • Increased material cost and longer curing times

For fasteners, maintaining the right thickness is especially challenging because threads and recesses are difficult to coat evenly. That is why clear specifications and systematic inspection are essential in purchasing and quality control.

2. Basic concepts: WFT, DFT and nominal thickness

Before looking at measurement methods, it is useful to clarify a few key terms.

  • Wet film thickness (WFT) is the thickness of the coating immediately after application, before solvent evaporation or curing. WFT is sometimes checked on-site with a simple wet film comb gauge during painting operations.
  • Dry film thickness (DFT) is the thickness of the coating after drying or curing. When we talk about coating thickness inspection for finished fasteners, we usually mean DFT.
  • Nominal thickness is the target thickness specified in the technical documents or coating datasheet. Standards like ISO 19840 define how to verify that the measured DFT values conform to the nominal DFT on rough steel surfaces.国际标准化组织+1

In practice, specifications are often given as a range such as 8–12 μm for zinc electroplating or 40–60 μm for certain zinc flake coatings, rather than a single number. Acceptance criteria will then define how many readings may fall below or above that range.

3. Overview of coating thickness measurement methods

Several methods are available to measure coating thickness. The choice depends on the coating type, substrate material, part geometry and required accuracy.

3.1 Magnetic induction

Magnetic induction gauges are the most common tools for measuring non-magnetic coatings such as paint or zinc on ferrous substrates like carbon steel. The probe generates a magnetic field and measures changes as it approaches the steel surface; the coating thickness is calculated from this effect.galvanizeit.com+1

Magnetic induction is widely used for:

  • Zinc electroplating on bolts, nuts and washers
  • Paint systems on steel structures
  • Zinc flake and other non-magnetic coatings on steel

Standards such as ISO 2178 describe this method for non-magnetic coatings on magnetic substrates.国际标准化组织+1

3.2 Eddy current

Eddy current gauges are used for non-conductive coatings on non-magnetic, conductive metals, such as paint on aluminium, or anodic coatings. A high-frequency alternating current in the probe creates eddy currents in the substrate; the effect of the coating on these currents is used to calculate thickness.defelsko.com+1

Combined magnetic / eddy current instruments are common. They automatically select the correct method depending on whether the substrate is ferrous or non-ferrous. ASTM D7091 covers both magnetic and eddy current techniques for dry film thickness measurement on metal substrates.ASTM International | ASTM+1

3.3 Ultrasonic

Ultrasonic gauges send a high-frequency sound pulse through the coating and measure the time taken to reflect from the substrate. This method is useful for thicker coatings and non-metallic substrates, and for measuring multi-layer systems where other techniques are not suitable.defelsko.com+1

For typical fastener coatings (zinc, zinc flake, paint on steel), ultrasonic methods are less common because the parts are small and the coatings are relatively thin. However, ultrasonic gauges become more relevant for thick protective coatings on structural components.

3.4 Destructive methods

Destructive methods include cross-sectioning (cutting through the coating and viewing under a microscope) and gravimetric methods (weighing before and after stripping the coating).defelsko.com+1

These methods are more time-consuming but can be used:

  • To verify non-destructive gauge readings during method development
  • When gauges cannot reach critical areas
  • For dispute resolution when measurement results are in doubt

Because fasteners are usually low-cost items produced in large quantities, destructive methods are generally used only for occasional validation.

For a practical overview of different measurement techniques and gauges, you can refer to:

4. Standards and guidelines for coating thickness inspection

Several standards provide detailed guidance on how to measure and evaluate coating thickness. Understanding these will help you set realistic and defensible requirements in your purchasing documents.

  • ASTM D7091 – Standard practice for nondestructive measurement of dry film thickness of nonmagnetic coatings on ferrous metals and nonmagnetic, nonconductive coatings on nonferrous metals. It describes important steps such as calibration, verification, adjustment and measurement procedures. https://www.astm.org/d7091-22.html ASTM International | ASTM
  • ISO 2178 – Non-magnetic coatings on magnetizable substrates – Measurement of coating thickness – Magnetic method. It specifies the magnetic method discussed above and the main factors that can affect accuracy. https://www.iso.org/standard/63004.html 国际标准化组织
  • ISO 19840 – Paints and varnishes – Corrosion protection of steel structures by protective paint systems – Measurement of, and acceptance criteria for, the thickness of dry films on rough surfaces. It provides guidance on instrument adjustment, sampling plans and acceptance criteria for rough steel surfaces.国际标准化组织+1
  • SSPC-PA 2 – Procedure for determining conformance to dry coating thickness requirements. It defines how many readings to take and how to evaluate them against specified ranges.KTA-Tator+1

As a buyer or QC engineer, you do not need to memorise every detail of these standards, but you should know which ones your suppliers follow and ensure that internal procedures are consistent with them.

5. Step-by-step procedure for coating thickness inspection

The following practical procedure works well for incoming inspection of fasteners and small metal parts. It is based on the principles in ASTM D7091, ISO 2178 and related guidelines, but simplified for warehouse and factory use.ASTM International | ASTM+2国际标准化组织+2

Step 1: Confirm the specification

Before measuring anything, clearly define:

  • Coating type (zinc plating, mechanical zinc, zinc flake, paint, hot-dip galvanizing, etc.)
  • Substrate material (carbon steel, stainless, aluminium)
  • Nominal coating thickness and allowed range
  • Relevant standards and project requirements
  • Sampling plan and acceptance criteria

This information should appear on drawings, purchase orders or quality agreements. If it is missing or unclear, clarify with the supplier first.

Step 2: Select the right gauge and probe

Choose a gauge that matches the coating and substrate:

  • Magnetic or combination magnetic/eddy current gauges for most fasteners and metal hardware
  • Eddy current probes for non-conductive coatings on aluminium and other non-ferrous metals
  • Ultrasonic gauges for thick coatings or non-metal substrates

Verify that the probe shape is suitable for small parts, curved surfaces and thread roots where necessary.

Step 3: Calibration and verification

According to ASTM D7091 and other standards, three operational steps are essential: calibration, verification and adjustment.ASTM International | ASTM+1

  • Calibration is usually performed by the manufacturer or a qualified laboratory. It sets the relationship between gauge readings and true thickness values on traceable standards.
  • Verification is done by the user before and after each measurement session, using certified shims or coated standards to check that the gauge reads within tolerance.
  • Adjustment may be necessary to account for substrate type, roughness or curvature by zeroing on an uncoated sample and/or using a shim of known thickness.

Keep records of verification results as part of your inspection files.

Step 4: Prepare the test surface

Ensure that test areas are:

  • Clean and dry, free from oil, dust or loose particles
  • At a stable temperature close to the gauge operating range
  • Accessible for the probe without rocking

On fasteners, avoid measuring directly on sharp edges or damaged areas. If you need to check threads, choose locations where the probe sits as flat as possible on the crest or flank.

Step 5: Take multiple readings

Coating thickness can vary across a part. Instead of relying on a single reading, take multiple measurements and calculate an average. Standards like SSPC-PA 2 provide guidance on the minimum number of readings per area.KTA-Tator+1

For incoming inspection of small parts, a typical approach is:

  • Select a sample size based on your lot size (for example, 10–20 pieces from each batch).
  • On each part, take several readings at different locations (for example, head, shank, thread area).
  • Record all readings, then calculate the average and identify minimum and maximum values.

Many modern electronic gauges can store readings, calculate statistics automatically and export data for reporting.诺沃测试+1

Step 6: Compare with acceptance criteria

Once you have a set of readings, compare them with your specification. Acceptance criteria may include:

  • Average DFT per part or per group of readings
  • Minimum value allowed for any individual reading
  • Maximum value allowed to avoid over-thickness

In some coating standards and paint specifications, rules such as the “90/10 rule” are used: a certain percentage of readings must be at or above the nominal thickness, while a small percentage may be slightly below, within defined tolerances.elcometerusa.com+1

If your project does not define such rules explicitly, it is good practice to agree simple criteria with your supplier in advance, for example:

  • Average of all readings within the specified range
  • No individual reading less than 80% of the minimum specified thickness
  • No reading more than 150% of the maximum, to avoid over-thickness issues

Step 7: Document results and communicate feedback

Prepare a simple inspection record that includes:

  • Lot or batch number and supplier name
  • Product description and coating type
  • Gauge model and serial number
  • Date of verification and reference standards used
  • All coating thickness readings and statistics
  • Pass/fail decision and comments

Share any non-conformities with your supplier promptly, including data and photos if possible. This helps them identify the root cause and improve process control.

6. Special considerations for fasteners

Fasteners present unique challenges for coating thickness inspection because of their shape and function.

6.1 Threads and fit

On threaded parts, coating thickness affects thread fit and torque-tension relationships. Excessive thickness can cause:

  • Difficulty engaging nut and bolt
  • Risk of cold welding or galling on stainless steel threads
  • Incorrect preload during tightening

When checking coated threads, it can be helpful to:

  • Use functional gauges such as GO/NO-GO ring and plug gauges in addition to coating thickness measurements
  • Focus on critical areas where the nut will engage, not just free length or the head

This combined approach gives a more realistic picture of whether the coated fastener will assemble correctly.

6.2 Small parts and blind areas

Some small fasteners or complex stampings have recesses that are difficult for gauge probes to reach. For these cases:

  • Use smaller probes specifically designed for small parts if available
  • Measure accessible areas that have similar coating conditions to the critical areas
  • Occasionally validate results using destructive methods such as cross-sectioning

Work closely with your supplier’s QC team to identify representative measurement positions during first article approval.

6.3 Different coating systems

Coating systems on fasteners include electroplated zinc, zinc-nickel, mechanical zinc, zinc flake, hot-dip galvanizing, organic topcoats and more. Each has its own typical thickness range and behaviour. For example, zinc flake systems can provide high corrosion resistance at relatively low thickness, while hot-dip galvanizing involves much thicker layers.galvanizeit.com+1

Discuss with your supplier which thickness range is appropriate for each system and how it affects performance, thread fit and appearance.

You can see examples of coated fasteners in product categories such as:

7. Integrating coating thickness control into your purchasing process

Coating thickness inspection is most effective when it is part of a broader supplier quality and purchasing strategy.

7.1 Clear specifications in RFQs and POs

Include key coating information in your RFQs and purchase orders:

  • Coating type and standard (for example, zinc electroplating with specific passivation)
  • Nominal dry film thickness range
  • Required test method and reference standard (for example, magnetic induction per ASTM D7091)
  • Whether inspection is required at the supplier’s factory, at incoming inspection, or both
  • Documentation requirements such as inspection reports or gauge verification records

This clarity helps suppliers design their process, select suitable coating partners and set up internal inspection routines.

7.2 Cooperation with supplier QC teams

If your supplier has its own QC team and inspection room, work with them to align procedures. Share your expectations on:

  • Gauge types and models
  • Calibration and verification schedules
  • Sampling plans and acceptance criteria
  • Reporting formats and key data you need

During factory visits, spend time in the inspection room to understand how coating thickness is controlled from raw material to final packing.

To learn more about manufacturer capabilities and QC processes, you can start from:

7.3 Risk-based inspection for different items

Not all parts require the same inspection intensity. A risk-based approach may include:

  • High-risk items (critical structural bolts, safety components, parts for coastal or marine environments): 100% supplier inspection plus incoming verification.
  • Medium-risk items (standard structural fasteners): routine supplier inspection plus sample checks at incoming.
  • Low-risk items (non-structural accessories): supplier inspection only, with occasional audits.

Adjust the level as you gain experience with each supplier and coating system.

8. Common problems and troubleshooting

Even with good procedures, coating thickness issues can still occur. Here are some common patterns and possible actions.

8.1 Batch consistently too thin

Possible causes:

  • Coating line process set incorrectly (short plating time, low current, insufficient spray or dip time)
  • Changes in coating bath composition
  • Incorrect or outdated work instructions

Actions:

  • Request detailed process records from the supplier for the affected batch
  • Ask for confirmation of bath analysis and corrective actions
  • Increase sampling on subsequent batches until stability is confirmed

8.2 Batch consistently too thick

Possible causes:

  • “Safety margin” applied by coater to avoid under-thickness
  • Out-of-date process parameters not adjusted after line changes
  • Inadequate control of coating viscosity or drainage

On fasteners, over-thickness can cause serious thread fit problems. In some cases, light rework (for example, controlled blasting or mechanical finishing) may be possible, but often the batch must be re-coated or rejected.

8.3 Highly variable results

If measurements vary widely across a batch or even on a single part, potential issues include:

  • Incorrect gauge use (tilted probe, unstable contact)
  • Poor surface preparation before coating
  • Non-uniform spray or dip application
  • Complex part geometry where coating builds up unevenly

Try to identify whether the variation is due to measurement technique or the coating process itself. Repeat readings with another trained operator and, if needed, validate using destructive methods.

8.4 Disagreement between supplier and customer measurements

Sometimes the supplier’s inspection report shows good results, but your incoming inspection finds under- or over-thickness. To resolve:

  • Compare gauges and verification standards used by both sides
  • Ensure both teams use the same measurement method and reference standard
  • Perform joint measurements on the same parts during a visit or video call
  • If necessary, send samples to an independent laboratory for cross-sectional measurement

Aligning procedures will reduce future disputes and help both parties understand the real performance of the coating.

9. Conclusion: turn coating thickness control into a strength

Correct coating thickness is a key factor in the performance and reliability of fasteners and metal components. By understanding the basic measurement methods, relevant standards and practical inspection steps, buyers and QC engineers can prevent many quality problems before they reach the project site.

A good coating thickness control system includes:

  • Clear specifications for each coating type
  • Suitable gauges and regular verification
  • Structured sampling and acceptance criteria
  • Strong cooperation between supplier and customer QC teams

When this system is in place, coating thickness inspection becomes a tool to build trust with your suppliers rather than a source of constant disputes.

If you are planning new projects or want to review the coating control on your current fastener portfolio, you are welcome to discuss your drawings and requirements with a manufacturer that operates its own inspection room and also integrates parts from partner factories to complete your list.

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