School is in… How polishing grades are measured

Staff at Global Stainless are expert in metal polishing offering finishes from dairy grade through to # 8 super mirror, and they know their stuff.

“We use a measurement called ‘roughness average’ (or Ra) to determine the surface grades of our polishing. An electronic instrument called a profilometer is used to check the polished finish against the required specification. Equipped with a tiny diamond needle which drags across the microscopic peaks and valleys of the polished surface, it calculates an average reading.”

Measurements are displayed on a screen in ‘μm’ (microns). One micron is one thousandth of a millimetre.

Very high mirror finishes are rated on the visual density of the very fine sanding grits marks that may be visible through the mirror polish finish.

It should be noted that ‘surface roughness’ differs from ‘surface finish’. The term surface finish is used to describe the appearance of stainless-steel plate or sheet and can be highly subjective. “Words like hairline finish, brush finish and matt finish are often heard in the industry. Surface roughness on the other hand is objective and is measured with a profilometer.

“There are some common surface finishes that our customers should be aware off. Come to Global Stainless and we can provide a service for all your polishing needs.”

Common surface finishes:

# 4 dairy polish – Standard polish for dairy industry and is achieved with buffing up to 180 grit and will typically measure between 0.4 and 0.7 Ra μm.

2B cold rolled plate – Most used stainless-steel sheet in the food industry with an Ra μm value between 0.3 to 0.5

# 6 dairy or sanitary finish – Sometimes the dairy or food industry will ask for buffing up to 240 grit which will produce a Ra μm reading of 0.3 to 0.5. This finish is also closer in Ra to 2B stainless steel sheet.

# 7 low mirror finish – This finish is produced using buffs up to 400 grit then soft mop buffed with polishing compound

This is Global Stainless’ specified economy finish, and it may appear with a slight haze due to the 400 grit sanding appearing through the mirror finish. It is used for lower cost mirror polished garden art. It has a typical reading of below 0.1 Ra μm.

# 8 mirror finish – Good mirror polish using buffs up to 800 grit. This is Global Stainless’ specified medium finish, below 0.025 Ra μm, it is used for high end mirror polished outdoor sculptures.

# 8 super mirror finish – Excellent mirror polish using buffs up to 1200 grit. This is Global Stainless’ specified premium finish. Used for indoor collectors’ artwork and ultra-critical hygiene pharmaceutical vessels.

True number # 8 mirror – Flawless mirror produced for the world’s elite fine art collectors.