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How to polish a knife to a mirror finish at home

How to polish a knife to a mirror finish at home

Introduction. Some history and purpose of polishing.

In the history of knife manufacturing, blade polishing has been used for a long time. It is safe to say that all knife lovers have seen and held various types of knives in their lives with polished blades of different kinds and quality.

Properly polishing not only gives the blade a rich and beautiful look, but is first and foremost a way to protect the steel from corrosion. The rule of thumb for any metal surface is that the cleaner you follow all the steps during polishing, the higher the finish quality of the surface and the less chance for tiny moisture particles to cling to the surface and cause it to corrode.

In ancient times, polishing was perhaps the only way to protect blades from corrosion, as all cutting tools were made of carbon steel. The highest skill of polishing from those times to this day is mastered by the polishing professionals in Japan. They have elevated their skill to a form of true art and the work on a single blade can take several days or more, depending on the size, given that it is exclusively manual work.

As of today, hardly anything has changed and no one is in a position to come up with something radically new.

To get a really high-quality and decent result even with using various modern devices, a polishing wheel and compounds, you also need to spend a lot of effort and have a certain amount of experience.

Methods of knife blade surface finishing

If you intend to give your blade a mirror finish by using the polishing method, or if you need to restore a blade edge after damage and to remove scratches from the bevels, then there are some technical aspects you need to understand first.

First of all, blade finishing is a very time and labor consuming process. Besides, not all steels, especially those hardened to high HRC values, have the same polishing properties. 

The polishing process itself is the final step after the blade has gone through the entire manufacturing process and grinding.

Grinding

Grinding is one of the most common ways of processing metal surfaces, both at manufacturing facilities, various workshops, and at home. The goal of this process is to get an even and smooth surface with a matte or mirror finish. To accomplish this goal, you have to follow certain steps.

The grinding process is divided into coarse, medium and fine steps. Grinding is a process that follows the forging and/or hardening of a knife blank in order to remove uneven areas and scales from the surface. During grinding, the workpiece receives the required dimensions and the desired shape of the bevels. All the necessary steps are carried out using a number of different electrical devices to speed up the production process. 

Grinding is a roughing stage of knife making and has many peculiarities of its own, which are difficult to describe within the framework of this article.  

The basic principle of this process is quite simple - the coarser the abrasive stone grain at the beginning of the process, the more time you will need to spend using a stone with a subsequent grain size to remove all of the scratches. 

The key to a quality grinding is the proper choice of the grit size of the grinding materials. An example would be grinding on a belt with the following sequence of grit sizes of belts: P24/36/40/60/80/120/180/180/240/280/400. After using this sequence, the transition to fine polishing will be much easier and you will get a quality result much faster.

Polishing

Often polishing is used in those areas where you need the best quality finishing of the metal surface, for decorative and aesthetic purposes, or if you need to protect the surface from corrosion and various weather conditions. In such a case, the metal surface gets polished to a mirror like finish.

Polishing is a process that follows grinding and gives the metal surface a higher quality structural state, which in the industry is expressed as a roughness degree. Depending on the abrasive products, there is very little metal removal and no deformation of the workpiece.

Our focus, however, is on carbon and conventionally stainless knife steels, which can get a mirror finishing to increase their protection from moisture and corrosion. A mirror finish on the balde, is not only useful, but also gives a certain aesthetic look to the knife blade edge.

Mirror finishing helps to protect the blade from moisture in the natural environment and from exposure to various acids during cooking. This is where a really good quality knife blade polishing makes a big difference.

Polishing methods

The polishing can be done in two different ways. If we consider the results achieved with machine polishing and hand polishing, it becomes clear that these ways offer completely different results.

Machine polishing

Along with the speed of processing as the main advantage, machine polishing has a number of disadvantages.

Any electric equipment has a corresponding cost and also requires additional operating costs and the need for accessories and consumables that are much more expensive than manual polishing consumables.

Besides, the physical characteristics of different steels do not promise the same surface hardness over the entire blade area. Even at fairly low speeds, electric equipment equipped with a polishing wheel can "eat out" material in the softer areas of the steel structure. This finish will result in a very unpleasant and uneven shine on the surface of the blade. 

On top of that, if the blade has distinct edges at the transition between the bevel and the blade flat, it is hardly possible to protect these areas with a duct tape. There is a good chance to remove or round off these edges and permanently ruin the appearance of the knife blade.

If someone who has little experience were to look at a knife with the naked eye, they would find this circumstance insignificant or unnoticeable, but if you look at a knife under a microscope, you will immediately grasp the point.

You will face the same problem when polishing the bevel area at the heel of the blade, which is the most difficult to polish due to the difficult geometry and size of the polishing wheel. You can polish this area, If you use a small polishing wheel and attachments with mini motor powered hand polishers, but you may still get a quite noticeable difference in the surface finish quality. 

To follow the abrasive hygiene during machine polishing, you will also need to change the polishing wheel when you change the paste, which involves additional expenses.

In general, with machine polishing, even with variable speed control, you can easily make a mistake, because it is easy to remove the material quickly and not always where and how you want it to.

Only use this way when you are really confident in what you are doing.

Manual polishing

Of all the ways to polish the surface of a blade, hand polishing is the most difficult and time-consuming and will require you to spend a significant amount of time if you have some knowledge. Consider everything described below as a guide, but the result will ultimately depend on your experience gained through practice.

The most important difference between hand polishing and machine polishing is not the method itself, but the materials that are recommended to use.

Regardless of the abrasives you choose, hand polishing involves using lapping tools of various configurations and materials. The main specification of a lapping tool is its hardness and the flatness of its surface. A lapping tool can be made of steel, glass or any other material.

It is also worth mentioning what was and is still used by Japanese masters polishers. A polishing stone is their main material and tool. The lapping stone can be a small bar made of natural or artificial sharpening stone of a size and grit matching your purposes.  

The process consists of slow back-and-forth movements of a sharpening stone or a lapping stone with sandpaper in combination with a cooling liquid. Once you have removed the scratches left by the previous abrasive, you can take a fine abrasive.

A hard lapping stone or a bar made of a sharpening stone will give you a flat surface with a quality of finish many times better than that of a machine. If your knife blade has convex or concave bevels, you can use a lapping tool of a rounded shape. However, it is worth giving some thought to whether such a thing is worth doing at all.

Step-by-step guide of the polishing process

Start of the polishing process

The first thing to do as preparation is to separate the handle from the blade if possible, regardless of whether you are using machine or hand polishing.  If this is undesirable or impossible, wrap the handle in cloth and/or seal with painter's tape or insulation tape. This protects the handle scales or the folding knife mechanism from dirt and abrasive dust.

Polishing process

For the polishing process, we recommend choosing sandpaper from about 600 grit and finer: 800/1000/1500 etc. That is, as long as it takes for the scratches from the previous sandpaper grit to lessen. You have to make forward and backward translational strokes.

You have to choose a suitable polishing compound depending on the grit size of your abrasives. You can make a polishing compound yourself from polishing pastes and special oils. It can be difficult to create a paste on your own, which is why there are ready-made solutions available.

Change of grit

You should change the abrasive material and the polishing compound as you proceed. Most often, a small amount of polishing compound will be enough to achieve a good result. For example, a quantity of paste equal to the size of a regular matchstick will be sufficient, but if necessary, it can be increased. Keep in mind that you are working on a loose abrasive stone that crumbles with every movement of your hands or rotation of the polishing wheel.

You must also remember that when moving to the next fine abrasive and polishing compound or paste, you must change the polishing wheel to a new one and to remove dust and dirt left by the previous abrasive and waste polishing compound. Otherwise you may get some very unexpected negative results and may have to repeat some steps.

Finishing polishing

You can end the whole polishing process the moment you get a visual proof after cleaning the blade and you are satisfied with the result.

Example of the polishing effect: When polishing M390 MICROCLEAN - BÖHLER steel, already after processing with 600-800 grit sandpaper it gets very difficult to oxidise the steel for the application of a protective chemical layer, e.g. with an etching solution containing iron oxide and nitric acid, even when heated to 60-70°C. 

Therefore, if you polish your blade made of similar or less corrosion-resistant grades with abrasives up to FEPA 2000 - 3000 grit and get a mirror-like finish, you can be confident that you got a very good result.

Conclusion

The process of polishing a knife blade to a mirror finish, and in particular hand polishing as the highest quality polishing method, comes down to the use of a range of abrasives with a step-by-step grit change. You can choose between the machine polishing or hand polishing depending on your skill or vision.

To avoid unwanted outcomes, first try to polish a knife you do not mind messing up.  At least you will find out exactly what you are good at and what still needs some more practice. Do not feel bad if you do not succeed right away - this is not an easy process.

As a word of advice - remember that there are a number of steel grades that are easy to polish, but they are also very easy to scratch. Only a really hard steel is difficult to scratch.

There are many different types of mirror finish and therefore the final result depends a lot on the steel itself, the methods and tools you choose, and your experience.

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