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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.

Aluminum Oxide Sandpaper
Click on image above to enlarge.

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.

Garnet Sandpaper
Click on image above to enlarge

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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