Sandpaper for wood finishing

The sandpaper
 

Sandpaper or abrasives is the material to sand any surfaces to make a smooth, even and level surface. In the wood finishing the sandpaper is also used to do the highlight and rubbing the finish. Basically sandpaper is made of 3 main components that are: minerals, bond (coat) and backing. The properties of sandpaper actually determined by and depending on the type of his constituent components and how they are unified.

The diagram of sandpaper

Minerals is the abrasives part to rub and sand the surface.  The mineral is made form hard material that is grind-ed to make a grain form. There are available some materials that can use for make abrasives, the most widely used are: aluminum oxide or silicon carbide that is pulverized up to a certain size. The size of mineral will determine the grade of sandpaper that shows the roughness of the sandpaper. Grade of sandpaper ranging from 0 as the coarser and 2000 as the finer. There are different standard of sandpaper grade The US, Europe and Japanese. The detail about the grade of sandpaper vs. the size of mineral can be seen in the table of comparison of sandpaper grade below.

The comparison of sandpaper grade

Each type of mineral has different properties. Aluminum oxide is less sharp but more durable. Sandpaper with aluminum oxide is more suitable to do the primary sanding as the sanding for the raw wood. It is used to sand the wood panels or components out from the  machinery. While the silicon carbide mineral has a sharper edges, but it is more easily to be blunted. Sandpaper with silicone carbide mineral is more suitable for sanding at the final process of wood sanding. The silicon carbide is also the mineral to make the sandpaper for sanding sealer or top coat in the finishing process.
Backing or buffer is a substance where the abrasives mineral is attached. Backing for the sandpaper is usually made of cloth or paper with different thicknesses. The type and thickness of the backing will determine the strength and flexibility of the sandpaper. The more flexible of sandpaper is the less strength and less rigid the sandpaper. Sandpaper with the thick backing and rigid is more aggressive. It is suitable to do the calibration and to form the wood component. The sandpaper with thick backing is mounted at the wide belt sander to do the surface calibration to make wood panel.
Meanwhile, the flexible sandpaper is more suitable to sand the complicated parts that can not be sanded by flat sanding machines, such as the: carving, posh, curved, etc. The flexible sandpaper is used to fill the brush sander to sanding the uneven parts. 
Now there are scotch brite and sponge sand, the more flexible abrasives to sand the more difficult form. Scotch brite is the abrasives which is made from synthetic fibers which is coated with minerals. While sponge sand is a sandpaper with sponge as the backing. Scotch brite and sponge are very flexible to reach the inside of the carving or crevices that are difficult to be sanded by rigid sandpaper.
Minerals are attached to the backing with tie or bond. There are 3 types of sandpaper according to its bond, the resin bond sandpaper or glue or a combination of both. According how the minerals is attached to the backing there are 2 types of sand paper, the open coat and closed coat. Closed coat sandpaper is the sandpaper when the entire surface of backing is covered by the mineral. while in the open coat sandpaper, only a portion (40%-70%) from the surface of the backing is covered by the mineral. Closed coat sandpaper is suitable for sand the strong and hard material but the material does not tend to stick on sandpaper. This type sandpaper is mostly used to sand the surface of raw wood or veneer. While the open-coat sandpaper type is more suitable to sand the surfaces when the material tends to stick to the sandpaper. This open coat sandpaper is more suitable to sand the finishing material such as: paint, sealer or lacquer in the finishing process.
During the sanding operation, there is heat generated by the friction between the coating and the abrasives. The excess heat will make the coating to melt and tend to attached to the sandpaper. In the sandpaper sanding is added the additive agent to reduce the heat caused by friction between the sandpaper and sealer or lacquer. The material is called a sanding agent and it is the same material with a material that is added to the sealer for the same purpose. The materials that are widely used as sanding agent are the: zinc stearate and aluminum stearate.
There are also available the waterproof sandpaper that is designed to do the wet sanding. It is used waterproof bond and backing. Wet sanding is sanding process which use the liquid to reduce the heat arising during sanding process. The wet sanding is done when the more smooth and even coating surfaces is needed. The wet sanding is the first step in the rubbing process to produce a high gloss finish when the smooth and flat surface is needed.