Metal polishes and finishes

A brief overview

'Buffing' and 'polish' are terms that are often used interchangeably, this can lead to uncertainty for both the experienced and inexperienced fabricator. There are also a wide range of different finishes available which can lead to further confusion. To help clear up any misunderstandings, here is a brief overview of various terms you may come across and a brief explanation of each one.

Polish

Polishing is used to remove surface oxidation, enhance the appearance of metal, create a reflective surface, and prevent corrosion. Polishing evens out the surface of the metal by using abrasive belts or discs to remove pits, scratches, and mill scale. Polishing produces a lined or brushed finish with visible grit lines that vary depending on the grade of abrasive used.

Buffing

Buffing is performed using a rotating cloth wheel or mops with fine abrasive polishing compound to achieve a variety of finishes most appropriate for the intended application. Buffing is often performed after a refinement polish to remove grit lines and create a bright lustre finish.

Mill Finish

Mill finish refers to the surface texture and the appearance of stock metal immediately after manufacture by extrusion, drawing, or being passed through a rolling mill. The quality of finish and surface characteristics vary substantially between batches, manufacturers, and even between items produced in the same consignment.

Typical characteristics of mill finish include;

  • Feeling rough to the touch
  • Swarf remaining on the surface
  • Lacking lustre
  • Signs of oxidation and rust spots
  • Possible contamination from mill oil
  • Surface marks and scratches
  • May have marks running the full length of profiles and extruded shapes where the stock has passed through the die

Some manufacturing mills use polished rollers to increase the smoothness and lustre of the surface, others may perform an anneal cycle after hot rolling to produce a matte finish. This further processing only embellishes the mill finish; the stock will still not have a uniform surface and will remain unsuitable for decorative use without being polished, and buffed, as required.

Copper Wash

Copper wash coating is also often referred to as 'copper finish' or 'lightly copper coated'.

Copper washing is a surface finish applied to mild steel, most commonly weld mesh, by completing an electrolytic process cycle. First, the surface oxidation is removed from the mild steel, next the metal is submerged in a copper sulphate solution; the copper sulphate solution is an electrolyte that conducts electricity from one electrode to another.

The loss of electrons (oxidation) at the copper anode releases copper ions into the solution, these ions travel to the cathode where reduction occurs, (reduction is the gaining of ions). This action plates the copper ions onto the surface of the mild steel as a micro-thin layer. The thin copper wash provides some corrosion resistance and has the added benefit of acting as a catalyst during further welding processes.

Copper washing does not pose the same fabrication issues, such as the risk of 'welder's flu', as galvanised stock.

Copper wash weld mesh can be painted or sprayed with metal paints and preparations to enhance its aesthetic appearance and to further protect it from corrosion after fabrication if required.

Copper washed products should not be used in aquatic applications as the copper will leach into the water supply and poison fish and other animals in these habitats.

Electro-galvanisation, Electro-zinc, Zintec

Electro-galvanisation is an electroplating technique to bond zinc to steel to provide resistance to corrosion, wear, temperature, and rust.

The electroplating process is performed by running an electrical current through a saline/zinc solution containing a zinc anode and a steel conductor. This results in the steel being given a surface coating of ductile pure zinc that is typically 1.0 to 1.5 µ (microns) thick and weighs up to 60g / m2 on profiles.

Since the 1980s, cyanide free, alkaline zinc has been used in electro-zinc processes to provide a more environmentally responsible solution than in the past.

Typical acid bath composition parameters. Value g/L
Zinc 40
Total Chloride 125
Anhydrous Zinc Chloride 80
Potassium Chloride 180
Boric Acid 25

When electroplated products are cut, it is recommended to protect the exposed steel edge with zinc oxide or other metal paints to maintain the corrosion resistance to that part. Electro-zinc is recommended for internal use only.

Electro-zinc is aesthetically different to the bright spangle of hot dip galvanising; it has a brighter matte grey/blue surface with a very thin, consistent, and uniform coating that achieves comparable protection.

Mirror finish

'Mirror finish' defines a level of reflectiveness that is equal to that of a traditional glass mirror. Mirror finish is achieved by a process of fine pre-grinding of the surface to eliminate surface defects and imperfections followed by a buffing cycle using soft mops and polishing compound. (Some grit lines may still be apparent).

This process results in a very smooth, decorative surface that is easy to keep clean. Mirror finish products are most commonly used for food conveyor systems, architectural features, and domestic and commercial vanity mirrors – especially where user safety is a concern.

Circular Polish

Circular polish is commonly used on sheet metals to create a decorative repeat pattern of overlapping circles.

The circular design is created by polishing with fine grit to produce a finish that is smooth to the touch and similar in appearance to a brushed/satin finish. Light, directional grit polish lines are clearly visible in this distinctive design.

Circular polish products are often used in fabrications where a more decorative design style is desired.

Anodised Finish

Anodised finish is achieved through a process of electrolytic passivation to create an increased natural oxide surface layer.

The base metal is placed in an electrically conductive solution where the cations of the solution are drawn to the electrode with a profusion of electrons, and the anions are drawn to the electrode that has a deficiency of electrons. A 'micro coating' is created by this chemical reaction which provides a 'shell' to protect the surface of the metal from corrosion.

The anodised surface will not chip, flake, blister, or peel off as it is part of the metal. Anodised surfaces have good UV resistance, good resistance to solvents and can be easily restored to 'as new' condition by simple cleaning.

The surface of the base metal becomes harder with anodisation and has a matte finish that is visually similar to brushed finish products.

Black Steel

Black steel has a distinctive dark blue surface with a scaly, dark iron oxide surface coating.

Black steel is hot rolled at high temperature, above the recrystallisation temperature of 1700°C; once the steel exceeds this temperature it can be easily formed and shaped. This presentation of steel is cheaper than bright drawn stock as no further reheating or processing is required.

There is less control over the final shape and size during production as black steel shrinks as it cools.

Black steel is mainly used in welding and construction where precise shape and tolerances are not required. Black steel can be used in all applications that do not require galvanising.

Bright Drawn

Bright drawn steel is black steel that has been further processed. After the initial hot rolling process, the mill scale is removed by pickling and then placed in a cold reduction mill, this process reduces the steel to the desired thickness while cooling at room temperature. Cold rolling improves the surface condition, ensures the stock is kept below recrystallisation temperature, and enables a range of tempers to be achieved. The steel is then reheated to complete an annealing or tempering cycle. Bright drawing produces a steel with closer dimensional tolerances than black steel and can improve its strength by up to 20%.

Bright drawn steel is a silvery grey colour and will require further surface treatments after fabrication to produce the desired aesthetic appearance.

Brushed polish

Brushed polish is often also referred to as dull or satin finish.

The stock is polished using a 120-180 grit wheel or belt followed by a cycle of greaseless 80-120 grit compound or abrasive pads / belt. This produces fine, unidirectional grit lines. The surface of the metal looks slightly frosted or 'milky' with a dull matte sheen; the metal will have a surface roughness of around 0.5 to 1.5 µm.

Brushed finish has a distinctive moderate lustre with very fine grit lines that run parallel to the direction of brushing. This finish does not show fingerprints easily and is easy to keep clean due to the long, shallow grain.

Brushed finish stock is most often used for watch backs, household appliances, catering equipment, shop fittings and architectural panels.

Some metal stock sold as satin finish does not have a unidirectional grain, it has no recognisable pattern as it is produced through sandblasting rather than directional brushing, however, the terms brushed, satin, and dull polish are usually considered to mean the same type of finish.

Bright polished

Bright polish finish is produced by using a soft buffing mop and polishing compound to create a bright, reflective surface. The metal receives no pre-grinding, so is only applied to stock with a defect free surface. Bright polishing does not remove surface defects but enhances mill finish surface. This is more reflective than standard bright drawn annealed stock and is often used in architectural and catering applications, particularly where economy of cost is a consideration.

Dull buffed

Dull buffed metal is used where a lustreless but even finish is required, and where appearance is not a critical concern. The surface of the metal is brushed with a bristle brush or fine abrasives to produce a semi-dull sheen with a uniform grain visible. This finish is mostly used to enhance the appearance of mill finish stainless steel.

Galvanised, HDG

Galvanised steel or iron stock has been subjected to a multi-stage process to produce a product that is very resistant to corrosion; it is suitable for use in external applications that require resistance to extremes of weathering, temperature, impact, and abrasion.

The surface of the metal is gently ground to remove any rough patches, it is then degreased, rinsed, pickled, rinsed, submerged in flux, and then dried. Once the metal has completed the initial process of preparation, it is submerged in a hot bath of zinc and iron at between 435°C and 455°C. The zinc and iron become metallurgically bonded to the base metal to form four zinc/iron alloy layers that protect the metal.

Levels from surface to base Composition of layer Diamond pyramid number. (Hardness)DPN
Eta Zn 100% 70
Zeta Zn 94% Fe 6% 179
Delta Zn 90% Fe 10% 244
Gamma Zn 75% Fe 25% 250
Steel or iron base metal To specification of metal grade used To specification of metal grade used

Once the metal has been cooled the surface will have a distinctive appearance that is silver- grey, looks rippled and grainy, sometimes with slightly powdery deposits that look flaky, but will not actually flake off.

Some stock has a smooth, uniform surface and some has a crystalline pattern that looks very much like frost- this is called spangle. Although these patterns can look quite different, they both provide the same level of corrosion resistance, the difference is merely aesthetic.

Galvanised stock is also referred to as 'hot dipped galvanised' or HDG.

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