Surface Finish Thickness Chart

This surface finish thickness chart compares common coating and treatment references used on sheet metal parts, then shows how to estimate their effect on holes, outside dimensions, fits, and threads. Use the chart to prepare a drawing or RFQ; the controlling standard, approved coating product, and finishing supplier should define the final thickness requirement.

Surface Finish Thickness Chart

The table leads with the source context because a number without its governing specification can be misleading. 1 mil = 0.001 in = 25.4 µm.
Surface Finish or TreatmentThickness ReferenceApprox. milHow to Interpret ItFirst Dimensional Model
Powder coating, single-coat product example 60–80 µm 2.4–3.1 mil Example recommended dry film thickness from a polyester powder product data sheet; another powder, color, texture, primer, or two-coat system can require a different build Added outward; use one film thickness per coated surface for early clearance checks
Cathodic e-coat product example 10–30 µm 0.4–1.2 mil Product range published for Axalta AquaEC A400/A600; the approved electrocoat and performance specification control Added outward; coverage is often more uniform than spray coating, but recess coverage and throwpower still depend on the system
Decorative/protective sulfuric anodizing AA 5, 10, 15, 20, or 25 µm 0.2–1.0 mil ISO 7599 uses thickness classes; the class number is the minimum average coating thickness, not a loose process range Only part of the oxide grows above the original aluminum surface; one supplier reference uses about 30% outward growth for clear anodizing
Hard anodizing, MIL-PRF-8625 Type III 50.8 µm nominal when unspecified, ±20% up to this thickness 2.0 mil nominal This is the default under the cited military specification when the contract, purchase order, or drawing does not state another thickness; it is not a default for every hard-anodize order The specification states that dimensional increase equal to one-half the applied coating thickness can be expected per coated surface
Electrodeposited zinc on iron or steel, ASTM B633 5, 8, 12, or 25 µm minimum 0.2, 0.3, 0.5, or 1.0 mil Four service-condition thickness classes, SC1 through SC4; supplementary finish type and hydrogen-embrittlement requirements are separate decisions Added outward, but current density can make edge and recess thickness less uniform than an electroless deposit
Electroless nickel-phosphorus, ASTM B733 0.1, 5, 13, 25, or 75 µm minimum 0.004, 0.2, 0.5, 1.0, or 3.0 mil ASTM standard service conditions SC0 through SC4 define minimum thickness; phosphorus type and post-plate heat-treatment class must also be specified Added outward; the process can produce relatively uniform thickness on wetted surfaces when solution circulation is adequate
Hot-dip galvanized fabricated steel, EN ISO 1461 45–85 µm minimum mean thickness, depending on steel section thickness 1.8–3.3 mil Mean minimum for non-centrifuged steel articles; local minima are lower and actual coating can exceed the standard minimum Treat as added outward for clearance planning, but expect more variation than precision plating and review holes, threads, drains, and runs separately
Stainless steel chemical passivation No deposited coating thickness specified — ASTM A967/A967M defines chemical passivation treatments and effectiveness tests, not a dimensional coating class Do not apply a generic coating allowance; cleaning, descaling, or etching may still alter the surface microscopically
Brushing, grinding, bead blasting, or polishing No added film thickness — These are subtractive or texture-changing operations rather than deposited coatings Do not add a coating allowance; specify the finished dimension and surface requirement if material removal matters
Metric values control this page. Mil equivalents are calculated with 1 mil = 25.4 µm and rounded to the nearest 0.1 mil in ranges or to the displayed precision in standards tables. Converted values are reference equivalents, not tighter acceptance limits. The fastest design check is to identify whether the process adds a film, converts the substrate, removes material, or combines more than one of these effects. ShincoFab’s sheet metal finishing services page can help narrow the process family before the exact coating specification is released.

How Coating Thickness Changes Dimensions

For a finish that builds outward, use the following first-pass model: b = f × c Where:
  • c = measured coating or treatment thickness on one surface
  • f = fraction of that thickness that grows above the original substrate surface
  • b = outward dimensional build on one coated surface
Then: External size after finishing = external size before finishing + 2b Internal size after finishing = internal size before finishing − 2b The 2b term applies when two opposite surfaces affect the measured width or diameter. A one-sided step or height changes by b. For deposited paints and metallic coatings, use f = 1 as an early dimensional model. For anodizing, use the conversion/growth guidance associated with the specified process rather than assuming the full oxide thickness grows outward.

Dimensional-Build Examples

The examples below start with a 10.000 mm opening and assume both walls receive the stated finish uniformly. They illustrate arithmetic, not guaranteed post-finish dimensions.
Example FinishCoating cOutward Fraction fBuild per Surface bTwo-Surface ChangeEstimated Finished Opening
Powder coating 70 µm 1.00 0.070 mm −0.140 mm 9.860 mm
E-coat 20 µm 1.00 0.020 mm −0.040 mm 9.960 mm
Clear anodizing, supplier growth model 20 µm 0.30 0.006 mm −0.012 mm 9.988 mm
Hard anodizing 50 µm 0.50 0.025 mm −0.050 mm 9.950 mm
Zinc electroplating 12 µm 1.00 0.012 mm −0.024 mm 9.976 mm
Electroless nickel 25 µm 1.00 0.025 mm −0.050 mm 9.950 mm
Hot-dip galvanizing 70 µm 1.00 0.070 mm −0.140 mm 9.860 mm
Actual coating inside a small hole may differ from the deposit measured on an accessible significant surface. Powder can be thin in recessed Faraday-cage areas, electroplating varies with current density, hot-dip zinc can accumulate or drain unevenly, and MIL-PRF-8625 notes that small and tapped holes can receive anywhere from no anodic film to a full normal coating. Critical openings therefore need a masking, allowance, or post-finish sizing strategy rather than arithmetic alone.

Anodizing Thickness Classes

ISO 7599 Decorative and Protective Anodizing

ISO 7599 covers decorative and protective anodic oxidation coatings on aluminum, but excludes hard anodizing used primarily for engineering wear resistance. The AA class identifies the minimum average coating thickness.
ISO 7599 Thickness ClassMinimum Average ThicknessApprox. mil
AA 5 5 µm 0.20 mil
AA 10 10 µm 0.39 mil
AA 15 15 µm 0.59 mil
AA 20 20 µm 0.79 mil
AA 25 25 µm 0.98 mil

Do not select the AA class from thickness alone. Alloy, temper, appearance, dye, sealing, exposure environment, pretreatment, and acceptable rack marks also belong in the finish definition. For precision components, discuss the pre-anodize etch because substrate removal can offset part of the outward oxide growth.

MIL-PRF-8625 Type III Hard Anodizing

MIL-PRF-8625 Type III is a hard anodic coating for aluminum and aluminum alloys. Under the cited revision, when no other thickness is stated, the nominal coating is 0.002 in or 50.8 µm. For coatings up to 0.002 in, the specified default variation is ±20% unless otherwise stated.

That produces a default interval of:

  • Lower limit: 40.64 µm or 0.0016 in
  • Nominal: 50.80 µm or 0.0020 in
  • Upper limit: 60.96 µm or 0.0024 in

The same specification advises placing both machining and coated dimensions on applicable close-tolerance drawings. Its expected surface growth is one-half of the applied coating thickness per coated surface. A 50.8 µm coating therefore produces an expected outward build of approximately 25.4 µm per surface, before considering pretreatment removal and process variation.

Zinc Electroplating Thickness: ASTM B633

ASTM B633 covers electrodeposited zinc on iron and steel articles. The service condition sets the minimum zinc thickness; the supplementary finish type defines the treatment over the zinc. Do not specify only “zinc plated” when corrosion performance, color, electrical contact, or compliance matters.
ASTM B633 Service ConditionMinimum Zinc ThicknessApprox. milService Description in the Class Name
SC1 5 µm 0.20 mil Mild
SC2 8 µm 0.31 mil Moderate
SC3 12 µm 0.47 mil Severe
SC4 25 µm 0.98 mil Very severe
These are minimum zinc values on specified significant surfaces, not a promise of perfectly uniform buildup everywhere. Edge current concentration, shielding, recess geometry, racking, and supplementary treatments affect the delivered coating system. High-strength steel also requires an explicit hydrogen-embrittlement review; ASTM B633 states that steel above 1700 MPa tensile strength should not be zinc electroplated under that specification.

Electroless Nickel Thickness: ASTM B733

ASTM B733 classifies electroless nickel-phosphorus by phosphorus type, service-condition thickness, and post-plate heat treatment. All three fields can affect performance; thickness alone does not define the deposit.

ASTM B733 Service ConditionMinimum ThicknessApprox. milGeneral Severity Label
SC0 0.1 µm 0.004 mil Minimum or flash
SC1 5 µm 0.20 mil Light
SC2 13 µm 0.51 mil Mild
SC3 25 µm 0.98 mil Moderate
SC4 75 µm 2.95 mil Severe
Because electroless nickel is autocatalytic rather than driven by current distribution, it can follow complex wetted geometry more uniformly than electrolytic plating. The ASTM scope still conditions this on adequate solution circulation. Deep blind holes, trapped air, long narrow passages, masking, and rack orientation need supplier review. For a 25 µm deposit on both sides of a diameter, expect a first-pass diameter change of 50 µm or 0.050 mm. A finished 10.000 mm bore would therefore begin near 10.050 mm before plating if the full deposit reaches both walls and no post-plate machining is planned. The drawing tolerance must also accommodate coating-thickness variation and the substrate’s pre-plate size variation.

Hot-Dip Galvanizing Thickness: EN ISO 1461

For non-centrifuged fabricated steel articles, EN ISO 1461 minimum coating thickness changes with the steel section thickness. The values below come from the Galvanizers Association’s published summary of the standard.

Steel Section ThicknessMinimum Local CoatingMinimum Mean CoatingApprox. Mean mil
Less than 1.5 mm 35 µm 45 µm 1.77 mil
1.5 mm to 3 mm 45 µm 55 µm 2.17 mil
More than 3 mm to 6 mm 55 µm 70 µm 2.76 mil
More than 6 mm 70 µm 85 µm 3.35 mil
These are minimum local and mean values, not nominal precision-film targets. Actual hot-dip galvanized coatings can be thicker because steel chemistry, section size, surface condition, withdrawal, drainage, and reactive silicon or phosphorus content influence growth. Threads, close-clearance holes, overlapping surfaces, sealed cavities, venting, and drainage should be reviewed before fabrication.

Powder Coating and E-Coat Need Product-Specific Thickness

Powder and e-coat thickness should be tied to an approved product or coating system rather than a generic industry number.

An AkzoNobel Interpon powder data sheet lists 60–80 µm for the referenced product. A TIGER Drylac product family also publishes examples around 60–80 µm, while other textures, primers, functional powders, and two-coat systems use different builds. A drawing that says only “powder coat black” leaves the chemistry, gloss, texture, film thickness, pretreatment, cure, and performance target unresolved.

Axalta’s AquaEC A400/A600 data sheet lists a 10–30 µm coating-thickness range. Electrocoat is useful for complex conductive metal parts because the bath and electrical field can reach areas that line-of-sight spray may miss, but film build still depends on product, voltage, bath control, geometry, drainage, and throwpower.

For either process, specify dry film thickness rather than wet film thickness. When primer and topcoat are both used, identify the required thickness for each layer and the total system rather than reporting only one combined number.

Finishes That Do Not Use a Generic Coating Allowance

Stainless Steel Passivation

ASTM A967/A967M covers nitric, citric, and electrochemical passivation treatments and tests for removing free iron and confirming treatment effectiveness. It does not define a deposited dimensional coating comparable to powder, zinc, or nickel. A passivation callout should name the governing standard, treatment or approved process, and acceptance test where required.

Passivation is also different from electropolishing. Electropolishing intentionally removes material and smooths peaks, so a precision feature needs a finished-dimension requirement and a process-specific removal allowance.

Brushing, Bead Blasting, Grinding, and Polishing

These operations change texture by removing or redistributing surface material. They do not add a dry film thickness. If the part has thin walls, sharp edges, cosmetic grain direction, weld blending, sealing faces, or a tight flatness requirement, define the finished condition rather than assuming the operation is dimensionally neutral.

Surface Roughness

Roughness values such as Ra, Rz, or a machining finish symbol describe surface texture, not coating thickness. A 1.6 µm Ra requirement does not mean that a 1.6 µm layer has been applied. When both roughness and coating thickness matter, state whether roughness is measured before or after finishing and whether the coating’s own texture is acceptable.

How to Use This Surface Finish Thickness Chart

  1. Identify the substrate, grade, condition, and any heat-treatment sensitivity.
  2. Decide whether the requirement is decorative, corrosion-resistant, wear-resistant, electrically functional, chemically resistant, or a combination.
  3. Select the governing standard or approved product specification before selecting thickness.
  4. State whether the listed value is minimum, nominal, range, average, local, or total system thickness.
  5. Define significant surfaces and inspection locations. A measurement on an open flat panel may not represent a recess, edge, hole, thread, or welded corner.
  6. Calculate the first-pass dimensional build on each coated surface, including the anodizing growth fraction when applicable.
  7. Choose masking, pre-finish allowance, or post-finish machining for critical fits.
  8. State whether drawing dimensions and tolerances apply before or after finishing. Use the sheet metal tolerance chart to separate general fabrication tolerances from finish-sensitive dimensions.
  9. Confirm the sequence for forming, welding, hardware insertion, coating, machining, and inspection.
  10. Review the complete drawing and coating specification with the selected supplier before production release.

Design Checks Before Releasing a Drawing

  • Specify substrate material, grade, alloy or temper, and nominal thickness.
  • Name the finish process and governing standard, including revision when contractually controlled.
  • State coating type, class, service condition, color, gloss, texture, seal, passivate, primer, or topcoat as applicable.
  • Distinguish minimum, nominal, local, mean, and total system thickness.
  • Identify significant surfaces, permissible rack or contact marks, cosmetic faces, and coating-free areas.
  • Show pre-finish and post-finish requirements for bores, slots, pins, gasket lands, bearing seats, press fits, grounding points, and mating edges.
  • Mask or separately control threads when film build could cause interference. Do not assume every coating will enter internal and external threads equally.
  • Define inspection method and location when thickness is critical; magnetic, eddy-current, X-ray, microscopic, coulometric, and other methods have different applicability.
  • Check whether pretreatment, etching, pickling, blasting, or polishing removes substrate before the coating is applied.
  • Confirm that the cure or post-plate heat treatment is compatible with the material temper, adhesives, inserts, seals, and prior operations.
  • Decide whether hardware is installed before or after finishing. The selected sequence can affect hole size, electrical continuity, corrosion interfaces, and coating damage; review it with the sheet metal hardware insertion plan.
  • Define whether critical inspection occurs before or after coating, assembly, welding, and any touch-up.

Material-Specific Finish Considerations

Aluminum

Anodizing converts aluminum into aluminum oxide, so alloy chemistry and temper influence appearance, achievable coating characteristics, and dimensional response. High-copper or high-silicon alloys can behave differently from common wrought sheet alloys. If color uniformity, hardcoat performance, or close fits matter, provide the exact alloy and temper for the aluminum fabrication review.

Carbon and Low-Alloy Steel

Powder coating, liquid paint, e-coat, zinc plating, electroless nickel, and hot-dip galvanizing protect steel by different mechanisms. A thin zinc plate with passivate is not interchangeable with a much thicker hot-dip galvanized coating, and neither is interchangeable with an organic barrier coating. High-strength steel needs special attention because acid cleaning and electroplating can introduce hydrogen-embrittlement risk.

Stainless Steel

Passivation helps remove free iron contamination and supports formation of a clean passive surface; it is not a paint-like film allowance. Powder coating or plating on stainless is a separate coating system with its own adhesion and pretreatment requirements. For visible or corrosion-sensitive assemblies, align the finish callout with the chosen stainless steel fabrication route.

Common Problems Caused by Missing Thickness Requirements

Holes and Slots Become Too Small

A finish applied to both walls reduces the clear opening by approximately twice the outward build. Even a 25 µm electroless nickel layer closes a uniformly plated diameter by about 0.050 mm. Powder coat or hot-dip zinc can consume substantially more clearance and may vary locally.

Pins, Tabs, and Outside Dimensions Become Too Large

Two coated sides increase an external width or diameter by approximately twice the outward build. This can turn a slip fit into an interference fit or prevent a tab from entering a slot.

Threads Bind or Lose Electrical Contact

Coating changes pitch diameter and flank clearance, while some organic finishes electrically isolate the joint. Masking, thread allowance, chase-tapping after finishing, conductive contact areas, or a different process may be necessary. Post-finish tapping removes local protection and should be accepted intentionally.

Appearance and Performance Vary Across the Part

Sharp edges, deep recesses, welds, porous castings, overlapping seams, and poor drainage do not receive every finish uniformly. A flat test coupon can meet thickness while a functional recess, edge, or thread does not. Significant-surface and inspection-location callouts reduce this ambiguity.

The Tolerance Stack Is Assigned to the Wrong Condition

A drawing may dimension the bare metal while assembly requires the coated size, or it may specify a finished size that the fabricator interprets as pre-finish. State the inspection condition directly. General fabrication tolerance plus coating variation plus measurement uncertainty can exceed the available fit clearance.

What to Do When the Finish Closes a Critical Fit

OptionWhen It HelpsTrade-Off
Add a pre-finish machining or cutting allowance Coating build is repeatable enough and the feature should be protected Requires verified coating growth and a tolerance stack that includes both substrate and finish variation
Mask the functional surface A bearing seat, thread, electrical contact, gasket land, or precision fit should remain uncoated Leaves the substrate locally unprotected and adds masking labor and transition edges
Ream, hone, grind, tap, or machine after finishing Final size is more important than complete coating coverage Adds setup and cost; exposes substrate and may require approved touch-up
Select a thinner finish or lower thickness class The application permits a different protection strategy Can reduce corrosion, wear, appearance, or qualification performance
Use electroless rather than electrolytic deposition More uniform build is needed on complex wetted geometry Chemistry, cost, phosphorus content, heat treatment, and substrate compatibility still require review
Increase mating clearance or change the fit class The assembly can tolerate more movement or gap May affect alignment, vibration, sealing, appearance, or load transfer
Define a coating-free hardware strategy Threads or inserted hardware cannot accept film buildup Requires sequence control and corrosion planning at exposed interfaces
Validate with coated first articles or coupons The finish and geometry are difficult to predict analytically Adds lead time but produces evidence for final drawing allowances and inspection locations

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Thickness Chart

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Frequently Asked Questions

Find quick answers to the most common questions about how surface treatments affect sheet metal dimensions, from specific coating thicknesses and hole reductions to engineering drawing best practices.

How thick is powder coating on sheet metal?
It depends on the powder and coating system. One common polyester powder product data sheet specifies 60–80 µm, approximately 2.4–3.1 mil, for a single coat. Other textures, primers, functional powders, and two-coat systems can be thinner or thicker, so the approved product data sheet should control.
If both walls receive a uniform 70 µm dry film, the theoretical diameter reduction is 140 µm or 0.140 mm. Real small-hole coverage may be thinner or uneven because electrostatic spray does not deposit uniformly in every recess. Use the calculation for early clearance planning and confirm critical holes by masking or coated-part measurement.

There is no single Type II thickness. ISO 7599 decorative and protective classes include AA 5, AA 10, AA 15, AA 20, and AA 25, where the number represents minimum average thickness in micrometers. A military, architectural, cosmetic, dyed, or supplier-specific requirement may use a different callout structure.

MIL-PRF-8625 states that an increase equal to one-half of the applied coating thickness can be expected on each coated surface. A 50 µm hard anodic coating therefore adds approximately 25 µm outward per surface, giving about 50 µm change across a coated external or internal diameter. Pretreatment and actual coverage still need to be considered.
No. ASTM B633 electrodeposited zinc classes are 5–25 µm minimum, while the EN ISO 1461 mean minima shown here for non-centrifuged fabricated steel are 45–85 µm depending on section thickness. The processes, coating structure, uniformity, appearance, and suitable geometry also differ.
A bore that receives 25 µm on both walls closes by approximately 50 µm or 0.050 mm. Electroless deposition can be relatively uniform on wetted geometry, but blind holes and narrow passages require solution exchange and fixturing review.

It is not normally specified as a deposited coating thickness. ASTM A967/A967M defines chemical treatments and acceptance tests for a clean passive surface. If material removal from prior descaling, pickling, polishing, or electropolishing affects a tight dimension, specify the finished size separately.

State the condition explicitly. Functional fits, bores, threads, grounding areas, sealing faces, and mating features are usually easiest to control when the drawing identifies their required post-finish size or clearly provides both pre-finish allowance and final acceptance criteria.

Need Help Specifying Surface Finish Thickness?

ShincoFab’s engineering team can review your drawing, material, thickness, finish standard, coating system, color or texture, significant surfaces, masking, critical pre- and post-finish dimensions, hardware sequence, quantity, and inspection requirements. Send the CAD files and finish callout for a part-specific manufacturing and finishing review.

 

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