In This Article
How Each Process Works
Powder Coating
Powder coating applies a dry polymer powder to metal using electrostatic spray. The part is grounded, and the powder gun gives particles a negative charge — they're attracted and stick uniformly to the grounded metal. The part then goes into a curing oven at 180–200°C for 15–30 minutes, where the powder melts, flows, and cross-links into a continuous film. The result is a hard, durable, even coating with no drips or runs.
Wet Paint (Liquid Paint)
Wet paint is applied by spray gun, roller, or brush as a liquid — typically a two-component polyurethane (2K) or epoxy system for metal. It dries and cures through solvent evaporation and/or chemical cross-linking. Multiple coats (primer + topcoat) are common. The process requires careful surface prep, controlled environment (humidity, temperature), and ventilation for solvent fumes.
Anodizing
Anodizing is an electrochemical process for aluminium (and titanium, magnesium). The aluminium part is submerged in an acid bath (typically sulphuric acid) and made the anode in an electrolytic cell. Passing current through the bath causes controlled oxidation at the surface — converting the outer aluminium to aluminium oxide (Al₂O₃). This oxide layer is hard, corrosion-resistant, and integrates with the base metal rather than sitting on top. Type II (standard) anodizing produces 8–25 microns; Type III (hard coat) produces 25–100 microns.
Quick Comparison Cards
Powder Coating: Deep Dive
What Makes Powder Coating Different
The key differentiator of powder coating is the monolithic film it creates. Wet paint layers — even high-quality 2K systems — have solvents that evaporate during cure, leaving microscopic pinholes and shrinkage stress. Powder coating doesn't have solvents; what you apply is what you get, and the melt-flow process fills in any gaps. The result is a consistently thick, pore-free coating that wet paint can't match at the same cost.
Surface Prep Requirements
Powder coating is unforgiving about surface prep. The electrostatic application reveals every defect in the substrate — scratches, weld spatter, mill scale, and contamination will all telegraph through the final finish. Standard prep involves degreasing, iron phosphate conversion coating (for steel), and blast cleaning to Sa 2.5 (near-white metal). Aluminium typically gets a chromate or non-chromate conversion coating before powder.
Powder Types
- Polyester (standard): Most common type. Good UV, colour retention, flexibility. Suitable for outdoor/indoor applications. Cures at 180–200°C.
- Epoxy: Excellent chemical resistance and adhesion, but poor UV stability (chalks outdoors). Best for indoor electrical panels, pumps, industrial equipment.
- Polyester-Epoxy (hybrid): Balance of both. Common for general industrial parts.
- TGIC Polyester: Better edge coverage and outdoor durability. Standard for architectural and outdoor use.
- Polyurethane: High-end option for appearance-critical parts. Harder, better chemical resistance, premium cost.
Limitations of Powder Coating
Powder coating is not suitable for assemblies with heat-sensitive components — the 180–200°C oven cure will damage plastics, rubber seals, electronics, and pre-installed hardware. It also struggles on very thin-walled parts (under ~0.8mm) that can warp in the oven, and on complex internal cavities that the electrostatic gun can't reach (Faraday cage effect). Minimum batch sizes at most powder coat shops mean it can also be expensive for one-off parts.
Wet Paint / Liquid Paint: Deep Dive
Why Wet Paint Still Matters
Wet paint is often overlooked in favour of powder coating for metal work, but it has genuine advantages. It can be applied to assembled products with heat-sensitive components, to large structures that can't fit in an oven, and in the field for repairs. Two-component polyurethane (2K PU) and two-component epoxy are the professional grades used in industrial manufacturing — not the aerosol rattle-can paint most people picture.
The Primer Question
For steel and iron parts, wet paint without a primer is a shortcut that will fail. Epoxy primer provides adhesion and corrosion protection underneath the topcoat. A zinc-rich primer (ethyl silicate or epoxy-based) gives galvanic protection similar to galvanising. For aluminium, an etch primer promotes adhesion to the oxide layer. Never skip primer on metal going outdoors or into humid environments.
Wet Paint in Indian Conditions
Monsoon humidity is the enemy of wet paint application and early film formation. Applying 2K PU at 85%+ relative humidity will cause blushing (milky appearance) as moisture gets trapped in the curing film. Most quality paint shops have climate-controlled spray booths for this reason. Verify your finisher has proper environmental controls if you're ordering during June–September.
Limitations
- Solvent VOCs require ventilation and disposal compliance
- More skill-dependent than powder — runs, drips, and orange peel are common in low-quality shops
- Thinner coating for a given cost (multiple coats needed for equivalent powder protection)
- Longer cure time (full hardness at 72 hours for many 2K systems)
- Touch-up is easier than powder coating, which requires re-baking
Anodizing: Deep Dive
Aluminium Only — and That's Fine
Anodizing is exclusively an aluminium process (along with titanium and magnesium, but those are niche). If your part is aluminium and will be exposed to the environment, handling wear, or strict appearance standards, anodizing is almost always the right choice. Nothing else gives you the same combination of hardness, corrosion resistance, and dimensional stability.
Type II vs. Type III (Hard Coat)
Type II (sulphuric acid anodize) produces an oxide layer of 8–25 microns, is available in silver and dyed colours, and is the standard for consumer electronics enclosures, architectural trim, and general engineering parts. Type III (hard anodize) uses lower temperature and higher current density to produce 25–100 micron layers with Vickers hardness of 400–600 HV — comparable to hardened steel. Hard coat is used for wear-critical components, hydraulic cylinders, and precision sliding surfaces.
| Property | Type II (Standard) | Type III (Hard Coat) |
|---|---|---|
| Layer Thickness | 8–25 µm | 25–100 µm |
| Surface Hardness (HV) | 200–300 | 400–600 |
| Colour Options | Clear, black, many dye colours | Clear, black (limited) |
| Salt Spray (ASTM B117) | 336–1000+ hours | 1000+ hours |
| Dimensional Growth | ~50% into / 50% out | ~50% into / 50% out |
| Surface Roughness | Follows substrate | Slightly rougher |
| Cost Premium vs. Type II | Baseline | 2–4× more |
Dimensional Impact of Anodizing
This is the detail most people miss: anodizing adds dimensional growth to the part. The oxide layer grows approximately half inward (consuming aluminium) and half outward (adding to the surface). A 15-micron Type II anodize adds ~7.5 microns to the surface on each side — that's 0.015mm on a diameter or gap. For most structural parts this is negligible, but for precision bores, threads, and mating surfaces, you must design the machined dimensions to leave anodize allowance.
Masking: Areas that must remain bare metal (electrical contacts, press-fit holes, threaded features) must be masked before anodizing. Communicate this clearly on your drawing. Anodized threads are harder to chase but do diminish the conductive contact — for RF and electrical grounds, mask the contact area.
Alloy Suitability for Anodizing
Not all aluminium alloys anodize equally. The anodize quality correlates strongly with silicon, copper, and iron content in the alloy — high levels of these elements create inclusions that show as dark spots or dull grey patches in the finish.
- 5052: Excellent anodize quality. Bright, even finish. Good for Class 1 (architectural) anodize.
- 6061: Good anodize quality. Minor cloudiness possible. Widely used for structural anodized parts.
- 6063: Best architectural anodize quality. Bright, clear finish. Used for extrusions, window frames.
- 7075: Poor anodize quality. Uneven, mottled finish due to zinc and copper content. Avoid for appearance-critical anodizing.
- 2024: Very poor. High copper content creates black smutting and non-uniform oxide.
- Cast alloys: Generally poor to acceptable depending on silicon content. Not recommended for decorative anodizing.
Full Property Comparison Table
| Property | Powder Coating | Wet Paint (2K PU) | Anodizing |
|---|---|---|---|
| Compatible Materials | Steel, Al, cast iron, zinc | All metals | Aluminium only |
| Coating Nature | Applied film (organic) | Applied film (organic) | Conversion layer (ceramic) |
| Typical Thickness | 60–120 µm | 40–100 µm | 8–100 µm |
| Adhesion Mechanism | Mechanical + chemical | Mechanical + chemical | Integral (is the metal) |
| Corrosion Resistance | Excellent (barrier) | Good (barrier) | Excellent (barrier + galvanic) |
| Salt Spray Hours | 500–1500+ | 300–800 | 336–1000+ |
| Scratch Resistance | Good (2H–3H pencil) | Moderate (HB–H) | Excellent (Vickers 300–600) |
| UV / Outdoor | Excellent (TGIC polyester) | Good (PU) | Excellent (no UV degradation) |
| Chemical Resistance | Good | Good–Excellent (epoxy) | Excellent (acid/alkali moderate) |
| Heat Resistance | ~120°C continuous | ~100°C continuous | ~200°C (oxide is ceramic) |
| Colour Options | Excellent (full RAL) | Excellent (any tint) | Limited (clear, black, few dyes) |
| Metallic / Special Effects | Texture, metallic powders | Good | Satin, matte, bright |
| Dimensional Change | +60–120 µm per side | +40–100 µm per side | +4–50 µm per side |
| Repairability | Poor (need to strip & recoat) | Good (local touch-up) | Poor (must re-anodize) |
| Process Temp | 180–200°C cure | Ambient or 60–80°C force dry | Ambient bath temperature |
| VOC / Environmental | Near zero VOC | High VOC (solvent-based) | Acid waste (regulated) |
| India Availability | Excellent (every city) | Excellent | Good (major metros) |
| Relative Cost (per m²) | Low | Medium | Medium–High |
| Min. Batch | Often 5–10 kg powder minimum | One-off possible | One-off possible (racking cost) |
Which One Should You Choose?
| Use Case | Best Choice | Why |
|---|---|---|
| Outdoor steel enclosure | Powder Coating | Best barrier + UV, cost-effective for steel |
| Consumer electronics enclosure (Al) | Anodizing | Scratch resistance, clean metallic look, consistent tolerance |
| Structural aluminium parts | Anodizing or Powder | Anodize for precision fits; powder for colour flexibility |
| Large assembled structure | Wet Paint (2K PU) | Can't fit in oven; field application possible |
| Electrical panel / cabinet | Powder Coating (epoxy) | Chemical resistance, consistent grounding through mounting points |
| Marine / high-humidity | Anodizing (Al) or Powder (Steel) | Both offer high salt spray resistance |
| Prototype / one-off | Wet Paint | No minimum batch, fast turn, easy colour match |
| Wear-resistant sliding surface | Type III Hard Anodize (Al) | 400–600 HV hardness, no delamination risk |
| Budget outdoor steel bracket | Powder Coating | Best cost-performance for steel |
| Appearance-critical interior part | Powder Coating or Anodizing | Powder for colour; anodize for premium metallic finish |
The combination nobody talks about: For steel parts that need a premium appearance without anodizing (which doesn't work on steel), powder coat over an iron phosphate conversion coating is the workhorse. For critical corrosion environments, consider zinc-rich primer (wet) followed by powder topcoat — the zinc acts as a sacrificial anode while the powder provides the barrier.
Finishing in India: Lead Times and Suppliers
India's finishing ecosystem is mature but fragmented. Here's what to expect:
Powder Coating
Available in virtually every industrial area of every city. Lead times are typically 1–3 days from receipt of parts. Most shops run standard polyester powder in RAL colours. TGIC polyester, textured powders, and metallics are available in larger shops in cities like Pune, Chennai, Bengaluru, and the NCR. Expect minimum batch charges if you're sending fewer than 5–10 small parts — it's not worth setting up the gun for one bracket. Price range: Rs. 30–120 per kg for small batches; significantly lower for volume.
Wet Paint
Quality wet painting (automotive-grade booths, 2K systems) is available from automotive OEM suppliers and defence fabricators. General-purpose industrial painting is available everywhere. Lead time: 1–4 days depending on number of coats and cure time required. The gap in quality between a roadside painting shop and a proper spray-booth operation is large — specify "spray booth, 2K polyurethane, DFT minimum 80 microns" in your purchase order.
Anodizing
Anodizing plants are concentrated in cities with significant aluminium machining industries: Bengaluru, Pune, Hyderabad, NCR (Noida, Gurgaon), Ahmedabad, and Chennai. Type II anodizing in silver and black is widely available; coloured anodize (blue, red, gold) is available at specialist plants. Type III hard anodize is less common — plan for 4–7 days lead time and shipping. Price range: Rs. 80–250 per kg depending on type, colour, and quantity.
Design Tips for Each Finish
Designing for Powder Coating
- Include a hanging hole (minimum 5mm diameter) for racking — often at an edge or corner that won't be visible
- Avoid blind holes pointing down — powder pools and creates defects; thread after coating or mask the hole
- Sharp inside corners create thin powder build-up and potential peel points; maintain R ≥ 1mm on all inside edges
- Internal cavities (tubes, channels) may not coat uniformly — consider leaving them uncoated or using a flood coat
- Weld spatter must be removed before powder; it creates bump defects in the final finish
Designing for Wet Paint
- Drain holes at low points prevent liquid paint accumulation during spray and rust initiation during service
- Specify primer type and DFT (dry film thickness) on the drawing, not just "paint"
- Allow 72 hours of cure time before handling or shipping for 2K systems
- Mask threads and precision bores before priming — paint thickness in a bore can prevent bolt entry
Designing for Anodizing
- Leave anodize allowance on precision fits: for Type II, add 0.02–0.03mm per surface to bores; subtract from shafts
- Avoid deep blind holes that trap acid — they cause inconsistent anodize and surface defects
- Mark masking areas clearly on the drawing with a note: "MASK PRIOR TO ANODIZE"
- Specify the anodize class: "Anodize Type II, MIL-A-8625, Class 2, Black" is a complete callout
- Welded aluminium parts anodize with a distinct weld zone appearance — the weld becomes darker or lighter. Design to accept this or keep welds out of visible areas
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