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Sandpaper 101
Everything you’ve ever wanted to know about sandpaper but were
afraid to ask.
How Does Sandpaper Work?
Sandpaper works a lot like a saw, chisel, or any other cutting tool in your shop. The
particles on sandpaper are made up from a number of sharp edges that cut the wood the same
way a saw blade does. The only real difference is that sandpaper, unlike your saw,
can’t be sharpened.Sandpaper is Sandpaper, Right?
Not exactly. There are two different grades of sandpaper on the market; Commercial and
Industrial. The commercial grade is commonly available at hardware stores and home project
centers. The industrial grade is usually available only through industrial supply stores.
It’s made from higher quality materials and is designed to be used in the rigors of
the production line.
What is grit?
When talking about sandpaper "grit" is a reference to the number of abrasive
particles per inch of sandpaper. The lower the grit the rougher the sandpaper and
conversely, the higher the grit number the smoother the sandpaper. This make sense if you
imagine how small the particles on an 800-grit sandpaper would need to be to fit into a
1" square. Sandpaper is referred to by the size of its grit (i.e. 150-grit
sandpaper).
What’s grit sandpaper should I use?
The grit you use depends on what you are trying to do. |
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The following table is a general guide to the proper uses,
although this may vary from wood species to species.
| Grit |
Common
Name |
Uses |
| 40-60 |
Coarse |
Heavy sanding and stripping,
roughing up the surface. |
| 80-120 |
Medium |
Smoothing of the surface,
removing smaller imperfections and marks. |
| 150-180 |
Fine |
Final sanding pass before
finishing the wood |
| 220-240 |
Very Fine |
Sanding between coats of
stain or sealer. |
| 280-320 |
Extra Fine |
Removing dust spots or marks
between finish coats |
| 360-600 |
Super Fine |
Fine sanding of the finish
to remove some luster or surface blemishes and scratches. |
So what’s the difference between Commercial and
Industrial grades?
There are three main components to sandpaper; the abrasive grit, the backing material,
and the bonding agents. Industrial grade sandpaper uses higher quality components as well
as tighter manufacturing tolerances.
Abrasive Grit – Industrial grade sandpapers use abrasive grit material that
is stronger and less likely to break down or wear out. Higher quality grits are often very
finely graded to ensure consistency.
Backing Material – Commercial grade samdpapers tend to use kraft paper or
low-grade fabric as a backing material. Higher grades of backing material are often made
from fine cottons or polyesters.
Bonding Agent – The bonding agent is the glue that attaches the abrasive to
the paper’s backing. Lower grades of sandpaper are often made from hide glue, which
doesn’t hold up well with heat or moisture. Higher-grade bonding agents such as
phenolic resin are used for industrial grade sandpapers.
I’ve heard woodworkers talk about "Going through the
grits". What does this mean?
"Going through the grits" is not a reference to eating breakfast in Georgia! It
refers to the process of sanding a workpiece where a woodworker uses progressively finer
pieces of sandpaper to get a smooth finish. By going through the grits each progressive
piece of sandpaper removes the scratches from the previous piece. Skipping grits to save
time is not necessarily a good idea. You will often end up sanding longer just to remove
the scratches left by the previous grit. This is more important with harder woods like
maple than it is with softer woods like pine.When Can I stop Sanding?
In most cases you can stop sanding at 150 or 180-grit. If you plan to use a water-based
stain it’s best to sand up to a 200-grit sandpaper. This is because water based
stains can tend to pickup and magnify a scratch. It can also be a waste of time to sand
with very fine sandpaper. Sanding maple with 400-grit sandpaper, for example, will tend to
seal off the grain and prevent finishes from penetrating.
What’s the difference between "Open-coat" and "Closed-coat"
sandpaper?
Open-coat sandpaper has gaps and open spaces between the grits that helps prevent clogging
by giving the sawdust a place to go. Open-coat is most often used for woodworking. The
spaces in the sandpaper can cover 40-60% of the sandpaper’s surface. Closed-coat is
better for sanding metal and wood finishes but clogs easily with sawdust. |
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What are the different types of sandpaper used for?
There are four main types of sandpaper abrasives used in woodworking; Aluminum Oxide,
Garnet, Silicon Carbide, and Ceramic. The two used most often by woodworkers are Aluminum
Oxide and Garnet.
Aluminum Oxide is one of the most common woodworking abrasives. It exhibits a
high degree of friability. This means that when heat and pressure are applied it
fragments. This is highly desirable because when Aluminum Oxide fragments it creates new
sharp edges. This self-renewing property allows aluminum Oxide to last longer than most
other sandpapers.
Garnet is a natural abrasive that is still commonly used in woodworking. It is
not friable like Aluminum Oxide and tends to wear out a lot faster. This is not
necessarily a detriment since garnet will produce a smoother surface than aluminum oxide
of the same grit number. It will however cut slower than its stronger cousin will. Garnet
is also an excellent choice for a final sanding paper. It has the tendency to burnish, or
seal off a wood’s grain. This allows pigmented stains to penetrate woods such as
birch, pine, or lauan, more evenly.
Silicon Carbide is harder than both Aluminum Oxide and Garnet. This makes it
suitable for cutting harder materials such as metal, paint, plastic, and fiberglass.
Silicon carbide is also a friable abrasive but wood is not hard enough of a material to
fracture its surface so it will tend to wear out faster than Aluminum Oxide.
Ceramic is not a friable material. It is the hardest of all of the abrasives
commonly available. It is generally available in only the roughest grades for fast wood
removal in woodworking. The most common use for ceramic sandpaper is shaping and leveling
of wood. It is also one of the most expensive materials available. You are most likely to
see ceramic abrasives used on belt sander belts.
Their uses are as follows:
| Abrasive
Type |
Wood |
Metal |
Paint |
Fiberglass |
Plastic |
| Aluminum
Oxide |
X |
X |
X |
X |
X |
| Garnet |
X |
- |
- |
- |
- |
| Ceramic |
X |
- |
- |
- |
- |
| Silicon
Carbide |
- |
X |
X |
X |
X |
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