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Metal polishing

Metal polishing, also termed buffing, is the process of smoothing metals and alloys and polishing to a satin, bright, or smooth mirror-like finish. This is achieved by use of abrasives pads, belts and/or wheels with polishing compounds selected so as to achieve the desired effect.

The use of abrasives in metal polishing results in what is considered a "mechanical finish".

Metal polishing is often used to enhance cars, motorbikes, handrail, architectural metal applications, etc. Many medical instruments are also polished to prevent contamination in marks in the metals.

All metal will oxidize over time. The removal of oxidization (tarnish) from metal objects is accomplished using a metal polish or tarnish remover; this is also called polishing. To prevent further unwanted oxidization, polished metal surfaces may be coated with a wax, oil or lacquer. This is of particular concern for copper alloy products such as brass and bronze.

Mechanical Finish Designations

-- #3 Finish – Also called Grinding, Roughing or Rough Grinding
These finishes are coarse in nature and usually are a preliminary finish applied before manufacturing. An example would be grinding gates off of castings, deburring or removing excess weld material. It is coarse in appearance and applied by using 36-100 grit abrasive.

When the finish is specified as #3, the material is polished to a uniform 60 - 80 grit.

-- #4 Architectural Finish – Also called Brushed, Directional or Satin Finish
A #4 Architectural Finish is characterized by fine polishing grit lines that are uniform and directional in appearance. It is produced by polishing the metal with a 120 - 180 grit belt or wheel finish and then softened with an 80 - 120 grit greaseless compound or a medium non woven abrasive belt or pad.

-- #4 Dairy or Sanitary Finish
This finish is commonly used for the medical and food industry – almost exclusively used on stainless steel. This finish is much finer than a #4 Architectural Finish. Great care should be taken in removing the surface defects in the metal – like pits – that could allow bacteria to grow. A #4 Dairy or Sanitary Finish is produced by polishing with a 180 - 240 grit belt or wheel finish softened with 120 - 240 grit greaseless compound or a fine non woven abrasive belt or pad.

-- #6 Finish or Fine Satin Finish
This finish is produced by polishing with a 220 - 280 grit belt or wheel softened with a 220 - 230 greaseless compound or very fine non woven abrasive belt or pad. Polishing lines should be soft and less reflective than a #4 Architectural Finish.

-- #7 Finish
A #7 finish is produced by polishing with a 280 - 320 belt or wheel and sisal buffing with a cut and color compound. This is a semi-bright finish that will still have some polishing lines but they should be very dull. Carbon steel and iron are commonly polished to a #7 finish before chrome plating. A #7 finish can be made bright by color buffing with coloring compound and a cotton buff. This is a good way to keep polishing costs down when a part needs to be shiny but not flawless.

-- #8 Finish or Mirror Finish
This finish is produced by polishing with at least a 320 grit belt or wheel finish. Care should be taken in making sure all surface defects are removed. The part is sisal buffed and then color buffed to achieve a mirror finish. The quality of this finish is dependent on the quality of the metal being polished. Some alloys of steel and aluminum cannot be brought to a mirror finish. Castings that have slag or pits will also be difficult – if not impossible – to polish to a #8.

Applications

Buffing is commonly used in metal polishing of pressure cookers, cookware, kitchenware. Pipes which are used in pharmaceutical, dairy and water industries are buffed to maintain hygenic conditions and prevent corrosion. Buffing is also used in the manufacture of high-quality lighting reflectors and architectural metal applications such as handrail and guardrail.

Process

Generally buffing is a multistage process. First, abrasives -- which may be coated, non woven or woven -- are applied at high speed to remove surface defects like pits, nicks, lines and scratches from the articles to be buffed. Then mops of various materials are used to smooth the surface to be buffed. Lastly, cotton mops are used to give a mirror-like finish to the articles. As a general rule, coarse grit abrasives are used first and fine grit abrasives later. This gives a smooth finish by forming very thin lines that are not visible to the naked eye. Lubricants like wax, diesel fuel and kerosene are used as lubricating and cooling media during these operations. Sophisticated computer-controlled machines are available which do buffing of intricately shaped articles. Before the advent of such machines, buffing was done by hand.

When polishing brass, there are often minute marks in the metal caused by impurities. To overcome this, the surface is polished with a very fine (600) grit tape, copper plated, then buffed to a mirror finish with a special cotton mop (called an airflow mop). Care must be taken when using these mops, as injury may result from the object being pulled out of the polisher's hands.