It is called sandpaper for finishes to all that abrasive in the form of a sheet of paper that allows improving and smoothing any surface that can be found in a construction site or in home repairs. They can be extra coarse (40 grit) or superfine (1000 grit) and have a paper or cloth (emery cloth) backing. They can be dry or water. The important thing is to know how to choose them according to the work to be done and the necessary finish; In this note we will explain how to do it.
Choosing a finishing sandpaper
Selection criteria
There is no universal sandpaper that covers all needs. The sandpaper They can be used dry, water, emery cloth. They can be made of aluminum oxide, silicon carbide, or other abrasives. They can be coarse, intermediate or fine. Each of them responds to a specific need.
We can create a table that will serve as a guide for acquire the sandpaper for the most appropriate endings for each situation. But first let’s see what they are, roughly, the selection criteria.
In any work under construction or home repair there are three large groups of materials: construction (concrete, plaster and brick), wood (hard, soft and artificial) and metal (steel, aluminum, iron, bronze, etc.). On those groups of materials the task is always similar: first you need to level and smooth the surface with coarse abrasives. They are then brushed over with a medium abrasive and polished or finished with a fine one.
The sandpaper They are suitable for all the groups of materials described, although for metals there are emery cloths, which allow them to be worked with more comfort and safety, especially if the work will be carried out using machine tools such as bench lathes or boring machines. They are also particularly suitable for polishing, as they are more resistant to traction.
Within the group of sandpaper are those that are used dry sandpaper and water sandpaper. The former are suitable for materials such as hard woods and metals; in general, materials that do not release large amounts of particles. The water sandpaper, on the contrary, are especially indicated for sanding surfaces such as soft wood, plywood, MDF, fiber-easy, and also plaster or plaster walls and ceilings, which release a large quantity of particles.
Coarse or fine sandpaper?
There are manufacturers that offer the general public sandpaper with grades of 40; correspond to a grain size of about 1.5 millimeters. Are they really necessary for the work or the home?
Yes: Whether it’s preparing the surface of a wooden board or a wall, the coarse sandpaper They play an important role in both cases, since they greatly speed up these tasks. Any imperfections in the wall or any splinters in a wooden board are quickly removed, but without excessive scratching of the surfaces.
The fine sandpaper, on the other hand, offer a much smaller grain (in the order of 300 microns for example)
Aluminum Oxide or Silicon Carbide?
The abrasive must be chosen carefully, because according to the type the results will be different; a bad choice can lead to a poor or even counterproductive result.
The silicon carbide sandpaper They are indicated for very hard materials such as marble, stone, metals and already cured concrete surfaces. They can also be used on very hard woods.
On their side, the aluminum oxide sandpaper They find use in softer materials such as plasters, softwoods and plastics.
The silicon carbide sandpaper they present an abrasive substrate of sharp-edged particles, which loosen as they wear, thus exposing new particles to the grinding action. On the contrary, those of aluminum oxide are spherical particles that do not come off easily.
Dry sandpaper or wet sandpaper?
Few people fully understand the difference between dry sandpaper and wet sandpaper.
The difference between dry sandpaper and water sandpaper It lies in how they are used. A dry sandpaper It is used directly on surfaces. Its use is faster, but it generates particles that could remain suspended in the atmosphere, causing inconveniences and even respiratory problems if you work without personal protection elements such as chinstraps and goggles.
The water sandpaper it is used wet: the sandpaper is soaked in water and the sanding operation is carried out under these conditions; thus, the particles are trapped by the liquid and do not run the risk of being suspended in the air. It is a cleaner method, but requires cleaning of the surfaces after sanding, especially if it is plastered walls or ceilings, or soft woods, conglomerates, etc. They are suitable for soft materials such as putties, soft woods, paints
Some concrete examples of the use of sandpaper
In order to fully understand what the sandpaper selection criteria for construction or home finishes, we will see some specific examples of use, with a suitable selection and its justification.
Sandpaper for plaster wall finishes
Here you should buy coarse grit sandpaper (grade 40 is ideal to start with) and medium grit wet sandpaper (grade 80) and fine (120) for smoothing and preparation. Then, 240 grade sandpaper for sanding between coats of paint.
The first sanding can be done dry, since the mission is to remove the old paint peels, smooth the plaster and the wall repairs. Already in the later stages it is convenient to work with water sandpaper to prevent fine particles from remaining in suspension in the atmosphere. Finally, the superfine sandpaper (grade 240) that is passed after applying a coat of paint must be dry to avoid damaging or to run the paint if it is also water-based.
Sandpaper for finishing tables, chairs and other solid wood furniture
In the case of solid wood, it is possible to work with dry sandpaper. In this particular case —sanding for subsequent varnishing of a garden table— we chose dry sandpaper grades 40, 60, 80, and water-based sandpaper grade 120. It is convenient to use sandpaper for a sanding machine, since the work will be much faster and easier.
We start with the 40, 60 and 80 grade sandpaper, using the lower grade (finer) only when the work with the coarser sandpaper is finished. The surface should be smooth to the touch, with no splinters sticking out of the wood.
Successive layers of varnish are applied, letting it dry and sanding with 120-grade water-based sandpaper for each coat. Finally and optionally we can use a 200 grade water-based sandpaper for the final finish. In this case it is possible to use a finishing sandpaper called antifilling, which prevents the adhesion of soft particles between the grains of the sandpaper.
Sandpaper for finishes of metallic elements (aluminum boards, aluminum or steel openings, etc.)
Unlike other materials, metal parts achieve better results with the use of emery cloth. The only difference with respect to the sandpaper is the backing, which – as its name indicates – is a cloth and not a paper.
The fabric allows work to be carried out directly on machines (although it is also useful for doing it by hand). In the photo above we see, for example, the work of polishing a piece on a lathe. As the spindle turns, the part also turns, and a strip of emery cloth is used for polishing.
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