Sanding, whether by hand or machine, is a crucial stage in achieving a desired effect on a vast array of materials. Its most widespread use is in the roughing and polishing of surfaces in general, but it also finds wide application in tasks such as removing scratches and scratches (from walls, floors or furniture), sharpening (knives, blades, swords, etc.), micro-finishing and precision finishing (connectors, fiber optics, electronics), superfinishing and lapping, among others.
Although various types of sandpaper they may appear similar to each other, are often made of different materials, have a wide grain range, and are more suitable than others for certain surfaces.
Some time ago we presented an introduction to these various sandpaper classes, where we mainly examine its composition Y grain type. We can summarize these concepts in the following illustrations.
Types of sandpaper
at the time of acquire the sandpaper that best suits our project, this information will be very useful, since we will be able to know a priori which is the most suitable abrasive material, as well as the correct grain to choose according to the desired application.
However, we still need to learn other important details to make the smart purchase of our sandpaper. Those details are what we will deal with or delve into in this article.
1) What format of sandpaper should I choose?
For many hobby users, a sandpaper refers only to a rectangular sheet that is sold in the hardware store in the neighborhood or in a hypermarket. However, although it is the cheapest, this rectangle is only one of many formats where can we buy one sandpaper. There are others, so it is convenient to know them all, since the format is important, especially if we are going to sand by hand or by machine.
sandpaper formats
2) How many types of supports are there?
As we pointed out in the introductory article, the sandpaper They consist of two parts: the grainy surface or abrasive material, which we detail in the first figures of this article, and a medium or back, which is usually paper, cloth or one plastic movie, where we will address the latter later.
The fabric support It is especially indicated when a high degree of flexibility of use of the sandpaper is required. It is more durable than paper backing, offers better tear resistance, and tolerates continual flexing and bending during use. It comes in three different grades:
Lightweight: very flexible, used where surface smoothness and finish are more important than material removal. It is usually present in sandpaper in roll, sheet and band format. Ideal for finishing and where great flexibility and conformity is required, such as contour work on curved surfaces.
Medium: Stronger and stiffer than the former, it is used in a wide range of abrasive products, from coarse-grain stock removal to fine-grain finishing and polishing. Consistent productivity, good finishes and long product life are characteristics of abrasive products manufactured on such a backing, such as belts and discs.
Heavy: it is the strongest backing and is used on coarse grain products designed for high pressure operations for heavy material removal. It is most often used in band sandpaper Heavy duty industrial.
The cheapest supports are paper. The quality of paper varies according to its intended use and is divided into different strength grades. This is how he sandpaper supplied for use with power tools is generally stronger than that marketed for hand use.
The importance of paper support deserves further discussion, since there are two kinds of paper that serve as support for a sandpaper, whatever the format. This leads us to a further consideration such as the following.
3) Dry or wet sanding?
Regardless of the type of substrate, sanding tasks can be carried out dry, or -provided the material of the work piece allows it- wet, that is, with water or some other lubricant. The advantages of wet sanding and, therefore, from sandpaper to water there are several, since they provide:
- Greater ease and efficiency: reduced friction and slugging ensure a faster and more effortless sanding process.
- Higher performance: a smoother sanding process produces a more uniform surface finish.
- Greater security: Wet sanding reduces the inhalation of dust and potentially harmful particles that are always released in any dry sanding.
- Greater Durability: the abrasive lasts longer as it is subject to less wear and less heat build-up during sanding.
- Greater ease of storage: Since they are waterproof, they can be stored anywhere, regardless of whether they eventually get wet or come into contact with water.
so that one sandpaper be suitable for use with water, obviously both the support and the abrasive material must be water resistant. So how can we know if the sandpaper What are we choosing, is it suitable for use with water? In principle, the manufacturer should indicate the legend “Waterproof”, “Waterproof” or similar, on the printed side of the sandpaper. If it is not indicated and the sandpaper is very cheap, it is most likely not waterproof.
Another characteristic to distinguish a water-based sandpaper is the abrasive material color: the brown color generally indicates a sandpaper not suitable for use with water, while the black or gray color does have waterproof properties. Usually, conventional sandpapers whose abrasive material is aluminum oxide are not waterproof, while sandpaper Silicium carbide and those that have polyester backing yes they are and, therefore, they are suitable for both dry and wet sanding.
4) Is flexibility important?
Returning to the subject of supports, we must add that in addition to the two traditional supports of paper and cloth, the plastic film backing, either polyester or other exclusive property. Film support gives much greater flexibility than cloth, so sandpaper can be folded, rolled, or molded into any shape. However, it also has other advantages: it is resistant to punctures, tears or wrinkles, it does not cake and it allows both dry and wet sanding. The flexible sandpaper They last 15 times longer than conventional paper-backed ones and are widely used not only in the automotive, electronics and fiber optic industries, but also on any irregular or curved surface where conventional sandpaper could easily tear.
The video shows an application of this type of flexible products.
5) Open or closed grain?
Carpenters and those who work with soft or rubbery materials should pay attention to the grain density of the sandpaper to choose, since this parameter divides the sandpaper into two groups:
Closed grain sandpaper: means that almost 100% of the sandpaper backing is covered with abrasive material. Therefore, the sandpapers of closed grain they offer a more aggressive and faster cut because the grain particles are densely packed together. They also provide the best finishes, since there are no gaps between the grains.
Open or semi-open grain sandpaper: it means that there is more space between the grain particles of the abrasive material. As usual, semi-open grain refers to a reduction of approximately 30% of the amount of grain in the support, while open grain This generally means that there is about 50% coverage. This means that these sandpapers offer less cutting power and are not as aggressive as one with a closed grain. But it also means that they have empty spaces in the layer of abrasive material, so the finishes they provide are not as even and are similar to those achieved with a coarse grit sandpaper.
So what grain density is appropriate?
For general jobs where the fine finish is important, it is always recommended to opt for a close grain sandpaper, unless we work with soft woods like pine, larch, cypress or cedar, for example, or with soft metals like aluminium, brass, bronze, copper, magnesium, titanium or zinc. If we use a closed grain sandpaper in these cases, the areas between each grain of the sandpaper will be loaded with waste, producing the phenomenon known as «filling«. This paste will not only create scratches on the surface of the material, but it can also ruin the sandpaper (either in paper, belt or disk format) and the workpiece itself.
Therefore, in the case of soft or even rubbery materials (eg plastic) an open grit sandpaper is very suitable. Since the grits are further apart, enough space is created between each grit which will help reduce the amount of sanding debris trapped between the grits. We can then use compressed air or a wand to dislodge the debris from the sandpaper and continue with the task. In this way, we can sand soft materials for longer periods of time and with better results.
Of course, in these cases we can also choose the anti-fill sandpaper.
6) How to deal with the filling?
If we are not convinced by an open grain sandpaper for soft woods, the anti-fill sandpaper offer even more benefits. Are sandpaper They are made with a special coating of zinc stearate, a water-repellent lubricating powder that ‘flakes off’ along with the residue during sanding, preventing clogging or cakeing of the sandpaper. These applications make use of its non-stick properties, leading to less sandpaper maintenance. Another benefit of this type of sandpaper is that the zinc stearate coating reduces the heat generated by friction, preventing damage to the surface of the material.
It is reasonable that the anti-fill sandpaper are a little more expensive than conventional ones and most are not waterproof, which is why they are still widely used today in the furniture industry, which was precisely the market niche they were intended for since their patenting some 60 years ago . In addition to wood, they are an excellent choice for dry sanding and finishing metal, plastic and other soft materials, where they show great performance and durability. They can be purchased in paper, roll or disc format with a grain size between 60 and 400.
7) Is there any super-sand that covers several demands at the same time?
As expected, the technology in terms of sanding accessories continues to advance and currently some manufacturers are already offering multipurpose sandpaper that meet many of the characteristics that we have seen and that highlight a very good relationship between material removal and useful life of the sandpaper. They feature an optimized aluminum oxide abrasive material, a paper backing with latex and a new anti-paste treatment, so these products achieve great flexibility while minimizing the paste, which makes them ideal for all types of applications on wood, especially to remove paint, lacquer, varnish or putty.
Once again, depending on our usual task of sanded or the project that we want to carry out, the correct selection of a sandpaper It is the big step to achieve and perfect results.
With this guide we have provided a series of additional elements that are not always taken into account when choosing a sandpaper, but it is worth considering carefully for the complete success of our work.
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