How to choose the correct abrasive
By Gene Wengert
360, 400, 600, 800, 2000); backing material and bonding agent
There are perhaps 500 different types of sandpaper when all the
possibilities are considered, but you can boil that down into the five
basic sandpaper options: Grit material (aluminum oxide, garnet,
silicon carbide or ceramic); grit size (typically 40, 60, 80, 120, 240,
(paper, cotton, polyester backing; hide glue or heat-resistant resins);
and open-coat or closed-coat. Let’s look at each option more closely.
Grit material: Four primary abrasive minerals for woodworking
are: aluminum oxide, garnet, silicon carbide and ceramic.
Aluminum oxide is a granular material that has very sharp
edges. If the mineral breaks when being used, new sharp edges
are formed on the remaining material. This ability to leave sharp
edges makes aluminum oxide sandpaper effective in removing wood
quickly and yet the paper is long lasting.
Garnet is a granular material, but it has more rounded edges,
Ceramic is an extremely hard abrasive, used for rapid wood
especially as it wears and the granular crystals break. Although this
round edge produces smoother finishes, it also means that sanding is
slower, and the paper tends to wear out faster. Because it rubs and
burnishes the wood more, it is an excellent choice for final sanding.
Silicon carbide is excellent for sanding hard materials such
as metal and plastics. It is commonly used on wet-dry sandpapers.
Wood fibers are too soft to cause the mineral to fracture and
develop new cutting edges, so this paper seems to wear out quickly
when used on wood. The most common use in woodworking is for
sanding (smoothing) finishes between coats.
removal, such as with belt sanders and abrasive planers. It is durable
and expensive, but it will not develop very smooth surfaces.
Grit size: The size of the abrasive particles determines the rate of
material removal and the smoothness of the resulting surface. Larger
sizes, or grits, remove wood faster but leave a rougher surface.
In general, under 80 grit is coarse and would be used for
high wood removal; 80 to 120 is medium and would be used for
moderate amount of wood removal and removing scratches left
by coarse sanding; 180 to 240 would be used to achieve a fairly
smooth surface before the wood is finished, although some people
prefer to go even finer (such as 320 or 360). Finer grits would be
used mostly for sanding between finish coats or for polishing.
Backing and bonding: The primary backing materials for
abrasives are paper, cloth and fiber. Paper, which comes in various
weights, is the most common for woodworking sandpaper. The
heavier paper weights are used with lower grits to provide the
needed strength; fine grits use low-strength paper, which is also
more flexible. Cloth backing, typically stronger than paper, comes
in various weights, with the heaviest weight used for the more severe
uses. Cotton fibers are used most often, but polyesters, especially for
the heavy weights, are also used.
Closed coat or open coat: If the abrasive particles essentially
cover the backing completely, then the paper is called “closed-coat.”
If there is space between the particles and only about 1/3 to 3/4
of the amount of abrasive is used per area, then it is called “
open-coat.” Open coat paper will not load as quickly as closed coat.
(Loading is the technical term for clogging.) Open coat would be
used for resinous woods, for example. Closed coats give smoother
Source: Gene Wengert, the Wood Dr., is a columnist for
Woodworking Network/FDMC magazine. Read more of his columns at
SANDING & FINISHING
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