What are the differences between kinds of sandpaper?
There are four different kinds of sandpaper, or abrasives that are used in the woodworking industry; Aluminum Oxide, Garnet, Silicon Carbide, and Ceramic. Most often, woodworkers will use aluminum oxide and garnet sandpapers.
Aluminum Oxide is one of the most common woodworking sandpapers. It has a high degree of friability. This means that when heat and pressure are applied it breaks up into smaller pieces, or fragments. This is a positive property of sandpaper because when aluminum oxide is used, it creates new sharp edges. (It's the sharp edges that scratch the surface of the wood. The bigger the pieces, the deeper the scratches. Smaller sharp pieces means a smoother finish) This allows Aluminum Oxide to outlast other sandpapers.
Many different manufacturers produce aluminum oxide papers and they are not all created equally, as there are different manufacturing processes that can be used to create them. If one brand of sandpaper does not fit your woodworking expectations, then it would be best to try a different company.
Garnet, the second most used sandpaper, is actually a natural abrasive (i.e. real sand) that is still commonly used in woodworking. It is not friable like Aluminum Oxide and tends to wear out a lot faster. Depending on what you are sanding, sometimes you do not want a sandpaper that is friable. Garnet will produce a smoother surface than aluminum oxide of the same grit number. It will cut slower than aluminum oxide however.
Garnet is also a good choice for a final use sand paper. It has the ability to burnish, or seal off a wood’s grain. This allows pigmented stains to be more evenly applied on woods such as birch, or pine. The downside to garnet (in lower quality sand papers) however is that the actual glued on grains of sand can come off and make an annoyance of themselves.
Silicon Carbide is good for cutting harder materials like as metal, plastic, and fiberglass since it is harder than aluminum carbide. Silicon carbide is a friable abrasive but wood is not hard enough of a material to fragment its surface so it tends to wear out more quickly than Aluminum Oxide. Unlike aluminum oxide, there is only one kind of silicon carbide. As it is good for sanding metal, you will find a better range of silicon carbide sandpapers at an auto parts store over a woodworking supplier.
Ceramic is the most expensive of the sandpapers, it is also the hardest of all of the sandpapers (not friable) commonly available. It is generally available in only the coarser grades for rough wood removal. The most common use for ceramic sandpaper is shaping and leveling of wood. Ceramic sandpaper is used primarily for sandpaper belts.
Lastly, besides the actual grit material used, is the backing paper itself. Normally a courser grit will be accompanied by a thicker paper. However, different companies will have their own variations and so depending on your needs it's best to check the stiffness of the paper as well.
There are two main types of sand paper backing, cloth and paper. Cloth can be heavy or light and paper backing is also subdivided into grades of letters, A through F. A being the lightest, and F being the heaviest.