Designing precision parts that still assemble correctly after coating is a common challenge for product developers. At Okdor, we’ve seen perfectly machined prototypes fail assembly when anodizing, plating, or powder coating adds unexpected thickness to critical surfaces.
Coating thickness typically ranges from 5-100+ microns, which can easily turn your ±0.05mm clearance into an interference fit. Threaded holes, sharp edges, and mating surfaces are especially prone to uneven buildup that disrupts assembly.
Learn how coating thickness impacts tolerances, which features need extra care, and when to adjust clearances to ensure proper fit after finishing.
Table of Contents
What Are the Main Types of Metal Coating and Plating Options?
Four main coating types are available: anodizing (creates colored aluminum finishes like on iPhones), electroplating (shiny chrome or nickel finishes), powder coating (thick, textured protection for outdoor equipment), and conversion coatings (invisible protection for basic rust prevention). Each provides different looks, protection levels, and costs for your product.
Working with product developers across aerospace, medical, and consumer electronics, we see the same coating confusion repeatedly. Most developers know they need surface treatment but get overwhelmed by technical specifications before understanding basic options.
Quick Selection Tool:
| I Want My Product To… | Choose This Coating | Examples | Thickness Impact |
|---|---|---|---|
| Look like premium electronics | Anodizing (aluminum only) | iPhone cases, audio gear | Minimal – won’t affect fit |
| Have shiny, decorative finish | Electroplating | Car trim, bathroom fixtures | Minimal – won’t affect fit |
| Survive outdoor/heavy use | Powder coating | Patio furniture, appliances | Thick – may affect tight fits |
| Just prevent basic rust | Conversion coating | Internal brackets, frames | None – invisible layer |
Cost Ranking: Conversion coating ()→Powder coating() → Powder coating ( )→Powder coating($) → Anodizing ($$$) → Electroplating ($$$$)
Understanding coating behavior helps avoid common mistakes. Anodizing creates an integral oxide layer that’s part of the aluminum itself, which is why it’s so durable on consumer products. Electroplating deposits actual metal layers, giving that mirror finish but requiring careful surface preparation. Powder coating flows and cures under heat, creating excellent coverage but building significant thickness that can affect assembly clearances.
For material compatibility, aluminum accepts all coating types well, while stainless steel works with everything except anodizing. When specifying coatings, consider ASTM B117 salt spray requirements for corrosion resistance and verify coating adhesion meets your application demands.
Design Takeaway: Use the table above to match your desired appearance and protection needs, then verify your material compatibility and dimensional constraints before moving to detailed specifications.
Which Metal Coatings Work on Stainless Steel, Aluminum, and Titanium Parts?
Aluminum accepts all coating types (anodizing, electroplating, powder coating, conversion), stainless steel works with everything except anodizing, and titanium is limited to specific electroplating processes, powder coating, and specialized conversion coatings. Material choice significantly impacts your coating options and performance outcomes.
Material-Specific Coating Compatibility:
| Metal | Anodizing | Electroplating | Powder Coating | Conversion Coating |
|---|---|---|---|---|
| Aluminum | ✅ Best option | ✅ All types work | ✅ Excellent adhesion | ✅ Chromate, phosphate |
| Stainless Steel | ❌ Not possible | ✅ Limited types* | ✅ Requires etching | ✅ Passivation common |
| Titanium | ❌ Not possible | ✅ Specialized only | ✅ Good adhesion | ✅ Anodizing-like process |
*Stainless steel electroplating requires special surface preparation and works best with nickel or chrome
From our experience manufacturing precision components, aluminum offers the most coating flexibility, which is why it’s popular in consumer electronics where both performance and appearance matter. Stainless steel’s chromium content makes anodizing impossible but provides natural corrosion resistance that often eliminates coating requirements entirely.
Titanium presents unique challenges – its oxide layer resists most coatings without aggressive surface preparation. However, titanium can be anodized through a different electrochemical process that creates colored interference films, popular in aerospace and medical applications where biocompatibility matters.
For multi-metal assemblies, powder coating often becomes the practical choice since it adheres well to all three materials with proper surface preparation. This eliminates the complexity of different coating processes for different components.
Design Takeaway: If you need anodizing’s premium finish and durability, design with aluminum. For mixed-metal assemblies, choose coatings that work across all your materials to simplify manufacturing and ensure appearance consistency.
What Design Features Should I Avoid When Specifying Metal Coatings?
Avoid holes smaller than 6mm diameter, sharp internal corners under 1mm radius, walls thinner than 1.5mm, and pockets deeper than 20mm without drain access. These features create coating defects, increase processing costs, and often require expensive part redesigns when prototypes fail quality inspection.
Coating-Unfriendly Design Features:
| Avoid This Feature | Specific Limit | Cost Impact | Design Fix |
|---|---|---|---|
| Small diameter holes | <6 mm diameter | 2–3× coating cost | Enlarge or add access holes |
| Sharp internal corners | <1 mm radius | Thick spots, defects | Add 1 mm+ radius minimum |
| Thin walls | <1.5 mm thick | Warping, uneven coverage | Increase to 2 mm+ or add ribs |
| Deep blind pockets | >20 mm depth | Pooling, runs | Add drain holes or reduce depth |
| Threaded blind holes | Any depth | Thread filling | Plan masking or post-coat tapping |
The most expensive coating failures happen when complex internal geometries prevent uniform coverage. According to ISO 12944 coating standards, thickness variation over 25% leads to premature coating failure, especially in corrosive environments. Features like narrow slots, deep recesses, and sharp transitions create exactly this kind of variation.
From prototype review experience, medical device housings with intricate internal channels often require specialized coating fixtures and multiple application passes, tripling standard coating costs. Similarly, aerospace brackets with sharp stress-relief notches frequently fail salt spray testing due to thin coating coverage at edges.
Think practically: if you can’t easily clean a feature with a brush or cloth, coating equipment will struggle to reach it uniformly. This principle applies whether you’re specifying powder coating, anodizing, or electroplating processes.
Design Takeaway: Use the dimensional limits above as design rules during CAD work. When functional requirements force you to exceed these limits, plan for masking, specialized fixturing, or post-coating machining to manage the added complexity and cost.
How Do Metal Coatings Affect Part Dimensions and Assembly Fit?
Plan for 10-100 microns of coating buildup on all surfaces when designing clearances and fits. The biggest design mistake is assuming coatings won’t affect assembly – they always do. Smart clearance planning during CAD prevents expensive redesigns when prototypes won’t fit together.
Design Clearance Planning:
| Your Assembly Type | Minimum Clearance Needed | Coating Recommendation |
|---|---|---|
| Sliding/moving parts | +0.15 mm beyond coating thickness | Anodizing or thin plating only |
| Press fits | Redesign or mask surfaces | Never coat press-fit surfaces |
| Close-fitting covers | +0.1 mm minimum | Any coating works with planning |
| Precision alignments | +0.05 mm or mask critical surfaces | Conversion coating or masking |
The most common design trap is creating assemblies that barely fit before coating. A 0.1mm clearance becomes interference after powder coating adds 75+ microns to both surfaces. This violates basic design principles outlined in ISO 2768 general tolerancing standards, where dimensional variations must account for all manufacturing processes.
For assemblies requiring tight fits, you have three design strategies: increase clearances to accommodate coating, specify masking for critical surfaces, or choose thinner coatings. Masking adds cost but preserves precise fits. Clearance adjustments are free but may affect product performance.
Consider which surfaces actually need coating protection versus appearance. Internal mating surfaces often don’t require coating for corrosion resistance, allowing you to mask them and maintain tight fits. External surfaces usually need full coating for durability and appearance.
Design Takeaway: Build coating thickness into your clearance calculations from day one. For critical fits under 0.2mm clearance, plan for masking or switch to thinner coating options like conversion coatings or electroplating instead of powder coating.
Can Metal Coatings Interfere with Threaded Assemblies and Electrical Connections?
Yes, but smart design planning prevents most interference issues. For threading, plan masking for critical fasteners or switch to self-tapping screws that cut through coating. For electrical connections, design dedicated contact areas or use conductive hardware that maintains grounding through coated surfaces.
Threading Solutions by Design Phase:
| Fastener Size | Design Strategy | When to Use |
|---|---|---|
| M3 and smaller | Always plan masking or use self-tapping | Critical assemblies, repeated use |
| M4–M6 standard | Open thread class or plan masking | High-stress connections |
| M8+ or coarse pitch | Usually works with post-coating cleanup | Lower-stress applications |
| Captive fasteners | Design with coating clearance built-in | When disassembly isn’t needed |
The threading challenge isn’t just about fit – it’s about assembly line efficiency. Coated threads that bind during assembly slow production and risk thread damage. ISO metric threading standards assume bare metal contact, so coating buildup requires thread class adjustments or post-coating operations to maintain proper engagement.
For electrical connections, the design choice is simple: create dedicated bare metal contact areas or accept electrical isolation and design around it. Star washers, conductive gaskets, and grounding straps work well when you need electrical continuity through coated assemblies. Many EMC compliance standards require verified electrical paths that coated surfaces can’t reliably provide.
Consider your assembly sequence too. Threading coated fasteners into coated holes requires different torque specifications and may need thread-cutting lubricants. Design for standard assembly processes when possible.
Design Takeaway: Size threading and electrical requirements early in your design process. Plan masking for critical M3-M5 fasteners, use self-tapping screws for non-critical applications, and create dedicated electrical contact zones rather than hoping coated surfaces will conduct reliably.
How Long Do Metal Coatings Take to Cure Before Assembly?
Most metal coatings are ready for assembly within 24-48 hours, but cure times vary significantly by coating type. Anodizing and electroplating are immediately ready, powder coating needs 24 hours for safe handling, while conversion coatings require 24-72 hours for full stability.
Assembly Timeline Planning:
| Coating Type | Cure Time | Assembly Ready |
|---|---|---|
| Anodizing | Complete during process | Immediate |
| Electroplating | Complete during process | Immediate |
| Powder Coating | 15–30 min cure + cool-down | 24 hours |
| Conversion Coating | Air dry 30–60 min | 24–72 hours |
Powder coating creates the biggest timeline impact because the thermosetting process continues after oven cure. Parts exit at 180-200°C and require controlled cooling to achieve full cross-link density. Handling them too early leaves fingerprints or adhesion issues that show up as coating failures later.
Temperature-sensitive assemblies need extra consideration – electronics can’t tolerate assembly onto parts that haven’t fully cooled. This thermal factor often drives coating choice in consumer electronics where rapid assembly is critical.
Anodizing and electroplating offer immediate assembly advantages since coating formation is complete when parts leave the process tanks. This makes them attractive for rapid prototyping and tight production schedules where coating can’t become a bottleneck.
Design Takeaway: Choose anodizing or electroplating when assembly timeline is critical, or build 24-48 hour cure time into your production schedule for powder coating. Never rush cure time to meet deadlines – field failures cost more than schedule delays.
Can You Rework or Remove Metal Coatings if Parts Need Modifications?
Most coatings can be removed, but it’s expensive, risky, and often damages the base material. The smart approach is designing to avoid rework needs entirely, but when unavoidable, some modifications work better than others.
Rework Decision Framework:
| Modification Needed | Best Option | Success Rate |
|---|---|---|
| Small holes (≤6 mm) | Drill through coating | High |
| Threading | Tap through coating | High |
| Large holes/cutouts | Remake part | N/A |
| Critical dimensions | Remake part | N/A |
| Surface defects only | Strip and recoat | Medium |
Chemical stripping follows ASTM B137 procedures but often leaves surface etching that affects recoat appearance. Heat stripping can alter aluminum temper properties, potentially voiding material certifications for structural applications. These process limitations explain why coating removal has 30-40% failure rates despite following standard procedures.
Coating removal typically costs 2-3x the original coating expense due to additional chemical handling, waste disposal, and surface refinishing requirements. For simple modifications like drilling or tapping, working through coating usually succeeds and maintains coating integrity around the modification.
Design Takeaway: Use the framework above to evaluate your options. Plan modifications before coating when possible, or choose conversion coatings if design changes are likely – they remove most easily without affecting base material properties.
How Do You Inspect Coating Quality on Complex Machined Parts?
Focus on functional defects that affect part performance rather than cosmetic perfection. Look for missing coverage on edges, coating runs that affect assembly, and adhesion problems. Most critical coating defects are visible without complex equipment.
Inspection Checklist:
- Edge coverage – Check sharp corners for bare metal (corrosion start points)
- Assembly interference – Look for coating runs on mating surfaces
- Adhesion quality – Press fingernail test on hidden areas per ASTM D3359
- Blocked features – Verify holes and threads aren’t filled with coating
Missing coverage on sharp edges creates the biggest long-term problems since corrosion starts at coating discontinuities. Coating runs or sags interfere with assembly clearances and should be caught before parts reach assembly.
For complex internal features you can’t inspect easily, establish process controls with your coating supplier rather than trying to inspect every hidden surface. If coating equipment can’t reach an area reliably, design changes work better than intensive inspection.
The most practical approach is creating a go/no-go checklist specifying which surfaces need complete coverage, acceptable defect levels in non-critical areas, and assembly clearance requirements.
Design Takeaway: Create a functional inspection checklist focusing on edge coverage, assembly clearances, and adequate adhesion. Communicate these criteria to suppliers rather than expecting them to guess what matters for your application.
What's the Most Cost-Effective Coating for Mixed Production Volumes?
Powder coating typically offers the best cost-effectiveness for mixed production volumes because it handles batch processing efficiently across multiple materials. You can coat aluminum, steel, and other metals together, spreading setup costs across entire batches for 30-50% cost savings.
Volume-Based Selection Guide:
- 100-1000+ mixed parts: Powder coating (best batch economics)
- 50-500 aluminum parts: Anodizing (no cure time, good for schedules)
- Under 50 parts: Conversion coating (minimal setup, basic protection)
The biggest cost driver is labor, not materials. According to industry coating standards, processes requiring extensive masking or multiple steps become expensive regardless of material costs. Powder coating’s straightforward batch processing delivers consistent value.
Cost reduction strategies:
- Design parts to nest efficiently in coating fixtures
- Eliminate masking needs through coating-friendly geometry
- Choose standard colors over custom matching
- Combine multiple projects in coating batches
Volume break-even analysis shows powder coating becomes most economical above 100 parts due to setup cost absorption. Below 50 parts, conversion coating’s minimal setup often costs less despite limited protection.
Design Takeaway: Choose powder coating for mixed-material production over 100 parts, anodizing for aluminum-focused runs, and conversion coating for small quantities. Design to minimize masking complexity for maximum cost efficiency.
Conclusion
Metal coating selection impacts every aspect of your product design – from material compatibility and dimensional tolerances to assembly processes and long-term durability. Smart coating choices during the design phase prevent costly redesigns and ensure your parts perform as intended.
Contact us to explore manufacturing solutions tailored to your metal coating and precision machining requirements.
Frequently Asked Questions
Specify thread masking for critical M3-M5 fasteners, use self-tapping screws that cut through coating, or open thread classes to accommodate coating buildup. Masking adds cost but ensures reliable assembly.
Yes, powder coating and most electroplating processes can handle mixed materials (aluminum, steel, stainless) in the same run. However, anodizing only works on aluminum, so mixed-metal assemblies requiring anodized finishes need separate processing or alternative coating strategies.
Conversion coatings at 1-5 microns provide basic corrosion resistance with minimal dimensional impact. For better protection, electroplating at 10-15 microns offers good durability while maintaining tight tolerances for precision assemblies.
Always coat after final machining unless you’re adding features that can cut through coating (like drilling small holes). Post-machining operations on coated surfaces often damage the finish and require touch-up or recoating.
Hard anodizing on aluminum provides excellent weather resistance with predictable 15-25 micron buildup. For other metals, consider thin electroplating followed by clear protective topcoats to maintain dimensional control while adding weather protection.
Add twice the coating thickness to your original clearance since coating builds up on both mating surfaces. For powder coating (75μm typical), increase clearances by 0.15mm minimum. Anodizing requires 0.04-0.05mm additional clearance.
