Your Polishing Questions Answered (Isue 1)

Mon 9th July 2018

Here we look back at some of the regular questions we are asked and the advice given by Kirsty Davies-Chinnock.

1. My customer has fabricated a sign out of mirror polished 316 stainless steel. It has been in situ for less than a month and it has started to discolour, why?

This is a really common problem and often comes as shock to those who are not used to working with stainless steel. On this occasion the grade and the finish were both correct and so the discolouration had to be from an external factor. One of the most usual issues of this sort of problem (believe it or not) is if the sign had been installed with mild steel screws / wall fixings. When this was eliminated I asked if there was any scaffolding near to the sign or any other repair work going on.

This was the answer. The re-fit of the premises was still underway and a variety of materials caustic to stainless steel – such as mild steel – were being cut nearby with an angle grinder. Tiny particles had adhered to the sign and it was these that had started to discolour. Fortunately the problem was solved very easily

2. We have welded a large tank and on removing the coating there is what appears to be pitting with some dusty debris inside the defects.

Weld splatter can go through protective tape and damage the surface – make sure it is protected. Again this is a very common complaint but is easily recognised by the splatter holes in the tape lining up with the defects in the surface – a much cheaper way of finding the problem than having samples tested in a laboratory.

3. Why can’t I have a ‘240’ grit on the kitchen cabinets I’m fabricating? Why must it be a ‘240 silicon’?

The short answer is you can have what you want so long as you are fully aware of the constraints of some finishes in particular applications. A 240 silicon adheres to the specification 2K on sheet under EN1008-2 and this has two important features particularly for catering applications (1) a roughness average on the surface of less than 0.5µm and (2) the use of silicon carbide as the polishing media which gives a ‘clean cut’ to the surface. Both of these aspects minimise cross contamination as they reduce areas where debris can sit. However of course an appropriate cleaning regime is essential whichever finish is decided upon.