Understanding Abrasives and Sanding Terminology

There's a lot of terminology in the woodworking world when it comes to abrasives and sandpaper. Read on to get the inside scoop on some of the more common terms and, hopefully, get help making more informed decisions about what abrasive products to purchase.

Abrasive grains cover 95-100% of the surface of the backing. Closed-Coat produces a more uniform scratch pattern but clogs more quickly than Open-Coat. It's a good good choice when sanding finishes and for sanding hardwoods such as Oak or Maple. It can provide more aggressive sanding and remove material quicker than Open-Coat.
Cloth Backing
Cloth backing is used on sanding discs and belts. Cloth is the stiffest and least flat backing material for abrasives. It will produce the coarsest and fastest cut. Cloth comes in two grades: J which is light and X which is heavy and less flexible. Cloth comes in varying degrees of these two grades as JF, J, XF, X, YX, and YY. JF through X are the lighter and more flexible of all the grades.
Film Backing
Polyester film backing, including Mylar, for abrasives look and feel like plastic. They are extremely flat, durable, and pretty stiff. Film backing is very smooth and even as opposed to paper backing which is made up of fibers and is irregular. Film backing abrasives give the most consistently even cut and at a faster rate than paper backed abrasives.
Friability means that the grains in sandpaper break down to expose fresh cutting edges as the sandpaper wears. Fresh cutting edges help the sandpaper to stay sharp and cut through more material longer.
Sandpaper grit refers to the size of the particles of abrasive material on the medium. Coarser grit material is used to take off more material faster while finer grits are used to smooth out the finished piece.
Most sandpaper for woodworking has an Open-Coat, meaning that abrasive grains cover only 40-70% of the backing material. The space between the grains give sawdust a place to go so the paper doesn't clog. open-Coat is better when less aggressive sanding is needed. It lasts longer than closed-coat sandpaper. Open-Coat sandpaper is good for sanding resinous softwoods such as white and yellow pine, and soft materials such as primer and topcoat paints.
Paper Backing
Paper backing is not as stiff as cloth backing but is flatter. It comes in grades A, C, D, E and F with A being the lightest and most flexible and F being the heaviest and strongest. Paper backing is generally used on sheet abrasives. The stiffer the paper, the less abrasive materials will deflect while cutting. They will cut deeper and faster. Softer backings will allow the abrasives to deflect more, giving light scratches and a smooth finish.


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