Types and grit size of sandpaper or sandpaper: sanding rules

Recommendation points

• Grits and numbers – how to determine graininess
• Skins by type of base
• Dry and wet sanding
• For manual processing
• Belts and discs for machine grinding

Sanding is one of the basic techniques for working with a wide range of materials. Processing, be it manual or machine, is carried out with abrasive paper. We will tell you about the numerous types of sandpaper and the principles of its choice in today’s review..

Grits and numbers – how to determine graininess

Grit, aka roughness, is a key parameter for any type of sandpaper. The grain size is always indicated on the back of the sanding paper after the letter P or the word Grit, sometimes both designations are used at once. The grain size is defined by a number from 12 to 15000, sometimes even more.

In its simplest form, this figure is the number of abrasive particles per square inch, if they are scattered in a continuous, even layer. In reality, this number is determined by the number of wires per square inch of the sieve through which the abrasive was sieved. The actual size of the particles ranges from visible to the naked eye (1-1.5 mm) to completely microscopic (whole and tenth microns).

Let’s define the area of ​​application of sandpaper depending on the grain size:

• up to P80 – for rough roughing and grinding in order to level the surface;
• from P100 to P220 – used at the second stage of grinding, if you need to eliminate small grooves and scratches;
• up to P280 – used for fine grinding;
• smaller skins are already classified as polishing.

You can read more about the exact choice of sandpaper for various purposes here.

The rule for choosing sandpaper by grit is very simple – the higher it is, the smoother the surface will be after processing. But at the same time, the finer the sandpaper, the faster it grinds down, and the removed layer of material becomes smaller. It should also be borne in mind that the greater the hardness of the processed material, the coarser paper can be used for finishing. At the same time, on soft wood, even with grit in P220, quite distinguishable scratches can remain..

Skins by type of base

Even in a small town, walking through hardware stores, you can find several dozen different samples of sandpaper. They will differ not only in grain size, but also in the method of applying the abrasive material, the type of filling and binder, as well as the used abrasive material or a mixture thereof. However, in practice, the type of substrate on which the abrasive is applied is of primary importance..

The cheapest and most quickly consumed emery cloth is produced on a paper basis. It has few advantages: in addition to its low price, paper is convenient if you need to quickly tear off a fresh piece of paper for work. Abrasive from such a skin crumbles rather quickly, especially in places of fracture, however, the paper base makes it easier to process embossed surfaces.

Cloth-based skin has a slightly higher cost, but much more durable. In many households, you can find half a dozen scraps of cloth sandpaper lying around, which have been successfully used from time to time for several years without losing their abrasive qualities. It also has some drawbacks: the fabric with epoxy impregnation is rough, under it the treated surface is less felt. Also, cloth sanding belts tend to stretch, although this mainly concerns only consumables for machining.

Finally, there is a third type of sanding paper – soft backed. This includes a foam or polyurethane cloth used for finishing embossed wood and plaster parts, and fiber sandpaper. The latter, although it has similar properties as pressing against the part, is used for attachment to a working body with Velcro, for example, on rotary grinders.

Dry and wet sanding

Depending on the abrasive material and its binder, the abrasive paper may differ in the acceptability of wet sanding. Besides the fact that wet sanding papers are more expensive than regular papers, there are a number of reasons why this distinction is important..

When removing particles from the treated surface, the friction forces at certain points can be so high that the generated temperature is sufficient for sintering metal dust. This, in particular, is true for aluminum and most non-ferrous metals: if the skin is not periodically shaken off, it will quickly clog up and become unusable..

In some types of paper, this problem is solved with a special abrasive material. So, silicon carbide, especially applied by electrostatics, is able to crumble, forming new cutting edges, so this paper practically does not clog. However, there can be really a lot of particles of removed material, for example, when processing plastic, and then it is necessary to prevent their adhesion by wetting sandpaper with water..

Polishing natural stone, marble or concrete is also not complete without the use of water or special compounds. Wetting improves sanding quality and prevents the spread of stone dust

Resistance to wetting is determined by the standard, which is indicated at the end of the marking on the back. Paper according to GOST 13344-79 allows work in a humid environment, but according to GOST 6456-82 it does not. There are exceptions, because water resistance is generally determined by the type of binder, that is, glue. Although the type of binder is usually not indicated, abrasives bonded with synthetic substances such as bitumen, polyester resins, phenolic varnishes, etc. are suitable for wet processing. Very often the possibility of using paper for wet works is additionally indicated by the letter “B” or the word Waterproof.

For manual processing

Often the home craftsman has to process his products by hand. So the quality of grinding is much higher, there are fewer untreated areas. For manual processing, paper is available in sheet, tape and roll format.

Tissue paper P60, P80 and P120 are the most popular types for grain size on the farm. The finer skin usually has a paper backing. It is recommended to always keep in stock different numbers of fine abrasive paper up to P400.

Cloth-based abrasives with a grain size greater than P300 are mainly produced for machine processing, although they can be handled with varying success. The main difficulty is that the abrasive is covered with a solid layer of binder, and processing with such a belt by hand is extremely slow, especially with high grain sizes. However, wet sanding with such paper is a pleasure..

Also, for manual processing, polyurethane grinding sponges will be very useful, with which it is very convenient to process parts with fine relief. If you are fond of woodworking, always have a supply of foam-rubber skin, this is the most effective preparation for painting or varnishing.

Belts and discs for machine grinding

When buying consumables for grinding machines, it’s hard to go wrong. All of them have a specific type and working dimensions – either length and width, or size number, or diameter.

For belt sanders and grinders, cloth-based paper rolled into a ring is used. Length and width in millimeters is the main parameter of such sandpaper, determined by suitability for use with one or another tool.

If you can still play with the width, using a narrower tape or tearing off the excess, then tapes of any length can only be used on grinders with adjustable tension. Also note that only one direction of travel is valid for the ring belt, indicated by the arrow on the back..

Abrasive paper for rotary and delta sanders is even easier. They either fit or not – the size of suitable consumables is clearly indicated in the equipment instructions. When choosing, you must also pay attention to the position of the holes for dust extraction. Since we are talking about a power tool, almost all the sandpaper in the consumables is designed for dry sanding. It remains only to choose the right grain size according to the type of material and the desired surface quality, and then gradually reduce it during processing.