Grinding and polishing: how to get a mirror
On stainless steel, grinding removes defects and sets the pattern, while polishing brings the surface to a reflection. A mirror is obtained not with one disc but with a chain of steps from coarse abrasive to paste: each next step removes the scratches of the previous one.
The main mistakes are skipping a grit, overheating the metal and working with a tool that was previously used to clean carbon steel.
Next — types of finish, the order of abrasives, the difference between №4 and a mirror and care for the finished surface.
Types of finish
Some finishes come with the rolled steel from the supplier, some are made after cutting, bending and welding. Rolled surfaces — 2B, BA, hot-rolled, calibrated — are formed at the mill. Local ones — grinding, №4, mirror — appear in the processing area when a seam has to be removed, an edge levelled or a visible part brought out.
| Finish | How it is obtained | Appearance | Where it is appropriate |
|---|---|---|---|
| 2B | rolling and annealing | matt, even | vessels, mechanical engineering |
| BA | rolling with gloss | light, smooth | decor, household appliances, medicine |
| №4 ground | grinding with abrasive | directional pattern | interiors, cladding, equipment |
| Mirror | polishing with paste | reflection without a pattern | visible elements, decor |
| Hot-rolled | hot rolling | dark, with scale | structures where appearance does not matter |
| Industrial | according to order requirements | varies | technical parts |
Before choosing a finish it is worth answering one question: will the surface be visible or working. For visible parts, uniformity of the pattern and the absence of streaks matter. For working and food surfaces — a continuous surface without stagnant zones that is easy to wash. A mirror solves the first task, but not always the second.
Abrasives step by step
Grinding proceeds sequentially. They start with a grit that removes the initial defects — scale, scratches, traces of welding — and finish with what gives the required class. It is impossible to skip steps: coarse scratches are not removed by a fine abrasive, they are only polished over, and under side light they show up again. The direction of movement is changed to perpendicular: then it is visible when the scratches of the previous step have been completely removed.
| Stage | Tool | Task | Control |
|---|---|---|---|
| Rough cleaning | flap or fibre disc | remove scale and the seam | no dark traces of heating |
| Levelling | medium-grit abrasive | remove the scratches of the previous step | change of direction of movement |
| Grinding | fine grit, №4 | set a uniform pattern | the pattern is even, without spots |
| Polishing | felt, wheel, paste | bring out the reflection | no streaks and "clouds" |
| Finishing | soft cloth, cleaner | remove paste residues | the surface is clean to the touch |
Two rules are more important than grit size. First — do not overheat: when grinding thin sheet the metal distorts, and heat tint means that the passive film is destroyed and the surface will have to be restored. Second — do not mix tools. A disc that has been used to clean carbon steel leaves particles of iron on stainless steel, and they give red spots that are easy to mistake for a defect of the grade. The overall picture of operations is in the article processing of stainless steel.
How №4 differs from a mirror
№4 is a ground surface with a directional pattern. It hides small irregularities, is simple to restore and is appropriate where a neat appearance and practicality matter. A mirror is a polished surface without a pattern that reflects objects. It requires an ideal base: any scratch or dent on a mirror is clearly visible.
| Feature | №4 | Mirror |
|---|---|---|
| Method | grinding with abrasive | polishing with paste |
| Appearance | directional pattern | reflection |
| Requirements for the base | medium | high, all defects are visible |
| Restoration | local re-grinding | more difficult, the whole plane has to be brought out |
| Service | hides scratches | emphasises fingerprints and scratches |
| Where it is chosen | interiors, equipment | decor, visible panels |
A mirror does not mean "better". For food surfaces a continuous clean surface is more important than the degree of reflection: fingerprints and streaks are noticeable on a polish, and it has to be wiped more often. What really affects hygiene and resistance, we analyse in the article is stainless steel harmful.
Care of the surface
A ground and polished surface lives longer if it is cared for correctly. The main rule is not to destroy the passive film and not to introduce foreign metal onto the surface. Chlorine-containing products, hard abrasives and steel brushes leave traces that then have to be removed again.
- wash with a mild product and water, without active chlorine;
- do not rub with an ordinary steel brush or a metal sponge;
- remove fingerprints and water drops before they dry;
- re-grind local scratches on №4 along the pattern;
- after cleaning and pickling, wash the surface with clean water;
- do not keep rags soaked in aggressive chemicals on the surface.
If red spots have appeared on the surface, first check whether they were introduced by a tool. Particles of ordinary steel are removed by cleaning and passivation, not by replacing the grade. How to obtain the required finish on rolled steel is visible from the items in the catalogue: for sheet and pipes the surface is stated in the order in the same way as in the certificate.
Frequently asked questions
How grinding differs from polishing
Grinding removes material with an abrasive and sets the pattern, polishing brings the surface to a reflection with paste. Grinding is a preparatory stage; without it polishing will not give an even mirror.
How to obtain a mirror surface
By successive steps of abrasive from coarse to fine with a perpendicular change of direction, then polishing with paste. It is impossible to skip a grit: the scratches will show up again under side light.
What the №4 surface means
It is a ground surface with a directional pattern. It hides small defects and is easily restored by local re-grinding. It is often taken for interiors and equipment.
Can scratches be removed from stainless steel
On №4 — yes, by local re-grinding along the pattern. On a mirror it is more difficult: the whole plane will have to be brought out, otherwise a noticeable spot will remain. Deep scratches are removed with a loss of thickness.
What to wash stainless steel with
A mild product and water. Chlorine-containing compounds, hard abrasives and metal sponges are not suitable. After washing it is better to wipe the surface so that no water spots remain.
Why streaks are visible on a polish
The causes are paste residues, a skipped abrasive stage or work with a dirty wheel. A full chain of steps and a clean tool, separate from the processing of carbon steel, helps.
Is it necessary to polish seams after welding
First the seam is cleaned of scale and heat tint, then if necessary it is pickled and only then brought out to the general class of surface. Polishing over scale is pointless: a depleted layer remains under it.
What to do next
A finish is easier to obtain on rolled steel with a predictable surface and even geometry. The surface is easier to bring out on rolled steel with even geometry. On the GCX exchange there is sheet, pipe, round bar and angle made of AISI 201, 304, 316L and 430; the finish and the grade are fixed in the order.