Learn about coating thickness standards for different surface treatments. Discover measurement methods, acceptance criteria, and documentation requirements.
Introduction
Coating thickness directly affects corrosion resistance, durability, and appearance. At Fulei Metal, we measure and control coating thickness to meet client specifications.
Why Thickness Matters
Too thin: inadequate corrosion protection, poor coverage, visible substrate. Too thick: cracking, flaking, wasted material, dimensional interference. Proper thickness ensures: optimal performance, consistent quality, cost-effectiveness, compliance with standards.
Thickness Standards by Coating Type
Powder Coating
General purpose: 60-80 microns. Architectural: 60-120 microns. Thin film: 30-50 microns. Maximum practical: 150 microns. ISO 9001 quality requirements.
Hot Dip Galvanizing
ISO 1461 specifies minimum thickness by material thickness. 1-3 mm steel: 45 microns minimum. 3-6 mm: 70 microns. 6+ mm: 85 microns.
Zinc Plating
ISO 2081 specifies: Fe/Zn 5 (5 microns) for indoor. Fe/Zn 8 (8 microns) for moderate. Fe/Zn 12 (12 microns) for outdoor. Fe/Zn 25 (25 microns) for severe.
Anodizing
ISO 7599 specifies: 5 microns minimum for indoor. 15 microns for outdoor. 25 microns for architectural. 25-75 microns for hard anodizing.
Nickel-Chrome Plating
ISO 1456 specifies: 10 microns nickel for indoor. 20 microns for outdoor. 0.3 microns chrome topcoat.
Measurement Methods
Magnetic Induction (Steel Substrate)
Measures non-magnetic coating on magnetic substrate. Common for powder coating, galvanizing, plating on steel. Accuracy: plus or minus 1 micron. Fast and non-destructive. Handheld gauges for field use.
Eddy Current (Non-Magnetic Substrate)
Measures non-conductive coating on non-magnetic metal. Common for anodizing on aluminum. Accuracy: plus or minus 1 micron. Fast and non-destructive.
X-Ray Fluorescence (XRF)
Measures coating thickness and composition. Works on multiple layers. Accuracy: plus or minus 0.1 microns. Non-destructive. Expensive equipment.
Coulometric Method
Dissolves coating electrochemically. Measures amount of dissolved metal. Destructive. Very accurate. Used for reference calibration.
Microscopy (Cross-Section)
Cut specimen, mount, polish, etch. Measure under microscope. Destructive. Very accurate. Shows coating structure.
Measurement Protocol
Frequency
First part: 100% measurement at specified points. In-process: sample measurement every 10-20 parts. Final: measurement on sample or 100% depending on requirements. Critical parts: 100% inspection.
Measurement Points
Flat surfaces: 3-5 points per significant surface. Edges: may be thinner or thicker. Corners: may be thicker. Recesses: may be thinner. Record actual values at each point.
Acceptance Criteria
Single point: may be below minimum if average meets spec. Average: must meet or exceed minimum. Maximum: no point exceeds specified maximum. Uniformity: variation within specified range.
Documentation
Thickness measurement records. Date and time. Part identification. Operator. Measurement device. Calibration date. Actual values. Pass/fail determination.
Common Thickness Issues
Uneven Thickness
Causes: poor application technique, part geometry, Faraday cage effect in recesses, improper electrostatic settings. Solutions: adjust application parameters, modify part design, use multiple passes.
Insufficient Thickness
Causes: too-short application time, low voltage, too-fast gun movement, worn nozzles. Solutions: increase application time, adjust voltage, slow gun movement, replace nozzles.
Excessive Thickness
Causes: too-long application, high voltage, slow gun movement. Solutions: reduce application time, adjust voltage, speed up gun movement.
Conclusion
Coating thickness control is essential for quality and performance. At Fulei Metal, our measurement protocols and documentation ensure coatings meet specified thickness requirements.
What Is Being Measured, by Which Method, and Where Readings Go Wrong
A thickness number is only meaningful with the method and the substrate stated. The table below pairs each measurement with the standard that governs it and the error that most often invalidates it.
| What is measured | Method and standard | Typical specification | Where readings go wrong |
|---|---|---|---|
| Dry film thickness on steel | Magnetic induction gauge, ISO 2178; measurement practice in ISO 2808 | Commonly 60-120 µm for a single powder coat | Measuring over a blast profile reads higher than the true thickness above the peaks |
| Dry film thickness on non-ferrous metal | Eddy current gauge, ISO 2360 | Same order, per the finish specification | Using a steel-calibrated gauge, or calibrating on the wrong substrate |
| Wet film thickness | Comb or wheel gauge, ISO 2808 | Used to predict the dry result before cure | Reading on a curved or textured surface, where the gauge cannot seat |
| Anodic coating thickness | Eddy current, with the coating type stated | Type II commonly 8-12 µm; Type III 25-75 µm | Measuring on a curved edge or on an unrepresentative area |
| Zinc coating on galvanized steel | Magnetic gauge or weighing, per ISO 1461 | Mass per unit area as well as thickness | Single readings on a coating that varies with section thickness and steel chemistry |
| Cure of a powder film | Solvent rub, such as ASTM D5402, or differential scanning calorimetry | Number of double rubs agreed in advance | Testing a thin panel whose thermal history differs from the thick sections |
Two habits make thickness data trustworthy. Take multiple readings across the part rather than one, because a single reading on a fabricated assembly is rarely representative; and state on the drawing whether the figure is a minimum on any single reading or an average, because the two are not the same requirement and the difference is a common source of dispute.
Frequently Asked Questions
Why do my readings differ from the applicator’s?
Usually gauge calibration on a different substrate, or measuring over a different surface profile. Agreeing the gauge, the calibration standard and the number of readings before production removes most of the disagreement.
Is more thickness always more protection?
No. Beyond the range the system was designed for, extra film increases cost and the risk of cracking, orange peel and poor flexibility. A well-applied coating at the specified thickness outperforms an over-thick one.
Where should thickness be measured on a part?
On flat, accessible, representative areas away from edges and recesses, with edges and recesses checked separately because they are deliberately the thin spots. A report that quotes one number for the whole part is not very informative.
Questions about a specific part are usually faster to answer against the drawing — send it through the route below.
About these figures. The reference values above come from our own production range; send the drawing for review lists the machines and materials behind them, and surface treatment quality inspection explains the adjacent steps that change the result. For your own part, surface finishing service is the fastest route to a quote — the earlier we see the drawing, the more of it can still be adjusted without cost. The tolerance context is set out in coating adhesion testing methods.