Surface Finishing Overview
Which finish to specify for laser-cut, machined and welded metal parts: coatings, plating, anodising and mechanical finishes compared
Surface finishing is everything done to a metal part after cutting, bending, machining or welding to change its corrosion resistance, appearance, hardness or cleanliness. For a buyer the practical question is which finish gives the required service life in the local environment at the lowest added cost. This overview compares the finishes available through the CutLinq network, with film thicknesses, typical salt-spray life and the materials each one suits, so you can write the right line on your drawing.
The finishes fall into four groups: organic coatings (powder coat, wet paint), metallic coatings (hot-dip galvanising, zinc plating, nickel), conversion coatings (anodising, passivation, phosphating) and mechanical finishes (deburring, brushing, blasting, polishing). Most parts need one from the mechanical group plus one from the others.
Finish comparison by thickness, protection and cost
The table below gives the numbers most often needed when comparing options. Salt-spray hours (ISO 9227) are laboratory values for red rust on steel or white corrosion on zinc; in Gulf coastal air, real life is roughly one year per 200-300 h for steel substrates. Relative cost is per square metre against powder coating of a simple flat panel set at 1.0.
| Finish | Thickness | Salt spray to red rust | Substrates | Relative cost |
|---|---|---|---|---|
| Powder coating, polyester | 60-120 µm | 250-500 h (phosphate), 1000 h (zinc primer) | Steel, galvanised, aluminium | 1.0 |
| Hot-dip galvanising EN ISO 1461 | 45-85 µm (steel 1.5-6 mm) | 1500-2000 h | Steel S235JR, S355JR | 0.8-1.2 |
| Electro zinc plating + passivation | 5-12 µm | 96-240 h | Steel, small parts | 0.4-0.6 |
| Zinc flake (Geomet type) | 8-12 µm | 600-1000 h | Fasteners, brackets | 0.7-1.0 |
| Anodising type II | 5-25 µm | 336-1000 h | Aluminium 5754, 6061-T6 | 1.0-1.5 |
| Hard anodising type III | 25-50 µm | 1000 h+ | Aluminium 6061-T6, 7075 | 2.0-3.0 |
| Passivation (citric or nitric) | no build | n/a, restores chromium oxide | 1.4301 (304), 1.4404 (316L) | 0.2-0.3 |
| Electroless nickel | 10-25 µm | 500-1000 h | Steel, brass, aluminium | 2.5-4.0 |
| Wet paint, 2-pack polyurethane | 80-200 µm (2-3 coats) | 500-1500 h (with zinc primer) | Large steel structures | 1.2-2.0 |
Organic coatings: powder and wet paint
Powder coating is the default for anything that fits in an oven (typically up to 6 x 2 x 2 m in the region's larger plants). It cures to 60-120 µm, comes in any RAL Classic colour and costs less per square metre than wet paint. Wet two-pack polyurethane or epoxy paint is used for structures too large for the oven, for site touch-up and where a dry film above 150 µm is specified, for example on oil and gas skids to ISO 12944 C4 or C5 systems.
Both need pretreatment: degreasing and iron phosphate for indoor steel, shot blasting to Sa 2.5 and a zinc-rich primer for coastal or industrial exposure.
Metallic coatings: galvanising and plating
Hot-dip galvanising gives the longest life per unit cost for structural steel: 50-85 µm of zinc alloy bonded to the steel, with 15-25 years to first maintenance in the dry inland climate of Riyadh or 10th of Ramadan City and 8-12 years in Jeddah or Alexandria coastal air. It distorts thin sheet below 2 mm and fills holes under 8 mm, so it suits frames, brackets and cable ladders rather than precision sheet parts. Electro zinc plating deposits only 5-12 µm; it is a bright, dimensionally accurate finish for fasteners, small brackets and machined steel, adequate indoors and short-lived outdoors. Nickel and chrome plating are decorative or wear finishes and are quoted per part.
- Galvanise: steel over 2 mm, outdoor structural, vent and drain holes of 10 mm minimum
- Zinc plate: machined and small sheet parts, indoor, tolerance-sensitive
- Galvanise then powder coat (duplex): 1.5-2.5 times the life of either alone
Conversion coatings: anodising and passivation
Anodising grows an aluminium oxide layer into the part: type II sulphuric anodising at 5-25 µm gives corrosion protection and accepts dyes (black, gold, red, blue, bronze); type III hard anodising at 25-50 µm reaches 400-500 HV for sliding parts and heavy sand abrasion. Dimensions grow by roughly half the coating thickness per side. Stainless steel is not coated; it is passivated in citric or nitric acid after machining or welding to remove free iron and restore the chromium oxide film, and welds are pickled to remove heat tint. Without passivation, 1.4301 (304) parts in a Dammam workshop show rust spots within weeks.
Mechanical finishes and surface roughness
Mechanical finishing sets the look and the roughness before any coating. Deburring is included in most laser and machining orders; the finishes below are ordered separately. Brushed stainless is specified by grit number, blasting by media and profile, and machined faces by Ra.
| Finish | Typical Ra | Description | Common on |
|---|---|---|---|
| As cut, laser | 3.2-12.5 µm on edge | Small burr on bottom edge, oxide on mild steel | All laser parts |
| Deburred, tumbled | 1.6-3.2 µm edge | Edge radius 0.2-0.5 mm | Sheet parts before coating |
| Brushed No. 4, 180-240 grit | 0.4-0.8 µm | Directional satin, hides scratches | 1.4301 (304) kitchen, fit-out |
| Mirror No. 8, buffed | under 0.1 µm | Reflective, shows every scratch | Decorative stainless, signage |
| Bead blast, glass | 1.0-2.5 µm | Uniform matt, hides tool marks | Aluminium before anodising |
| Shot blast Sa 2.5 | 40-75 µm profile | Bright grey, prepared for paint | Structural steel |
| Milled, standard | 1.6-3.2 µm | Visible tool paths | Machined aluminium, steel |
| Ground | 0.2-0.8 µm | Flat, precise | Shafts, sealing faces |
Design tips
- Write the finish as a specification line: process, thickness, colour or grit, and standard, for example 'Powder coat polyester RAL 7035 satin, 80 µm min, ISO 2808'.
- Allow for coating build in fits: 0.08-0.12 mm per side for powder, 0.01 mm for zinc plating, half the layer thickness for anodising.
- For galvanising, add vent and drain holes of at least 10 mm in every closed section and avoid sheet under 2 mm.
- Keep anodised assemblies within one alloy and one batch; 6061-T6 and 5754 dye to visibly different shades.
- Specify brushing direction on the drawing (long axis) for stainless panels that will sit side by side.
- Mask threads and bearing seats only where necessary; each masked feature adds 1-3 minutes of labour per part.
- For Gulf coastal sites, plan a duplex system (galvanise plus powder) or 316L stainless rather than painted mild steel.
Applications
| Industry | Example parts | Typical material |
|---|---|---|
| Solar EPC | Mounting structures, purlins | S235JR hot-dip galvanised 60 µm |
| Food and beverage | Tables, hoods, conveyors | 1.4301 (304) brushed 240 grit, passivated |
| Signage and facades | Letters, screens, fins | 6061-T6 anodised 15-20 µm, DC01 powder coated |
| Electrical | Enclosures, DIN plates | DC01 powder RAL 7035, DX51D+Z zinc plated |
| Machinery OEM | Guards, guides, fixtures | S235JR powder coated, 6061-T6 hard anodised |
| Oil and gas | Skid frames, walkways | S355JR blast Sa 2.5, epoxy-PU 240 µm |
| Furniture and fit-out | Frames, brackets, handles | S235JR powder matt, CuZn37 lacquered |
Pricing and lead time
Finishing cost depends on three things: surface area and part geometry (hollow frames and assemblies cost more per kilogram than flat panels), the system specified (single-coat powder is the baseline; duplex galvanise plus powder costs roughly 2 times, hard anodising roughly 2-3 times, electroless nickel 3-4 times) and batch size, because line set-up, colour changes and jigging are fixed costs spread over the parts. Typical added lead time is 2-4 working days for powder coating or zinc plating, 3-5 working days for hot-dip galvanising and anodising, and 5-8 working days for two-coat paint systems or hard anodising, on top of cutting or machining.
Frequently asked questions
Hot-dip galvanising for structural parts, or galvanise plus powder coat if colour matters. For sheet under 2 mm, use pre-galvanised DX51D+Z with a powder topcoat, or move to 1.4404 (316L) stainless for parts expected to last over 10 years.
Upload your DXF or STEP file with the finish line from your drawing and receive a binding quote for fabrication and finishing within 12 hours.