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Sandpaper is meant to be friable. The aggregate starts to shatter as its used, the broken surfaces are sharp, and the belt keeps cutting as good as new ( until its worn out ). If you buy automotive belts ( for instance ) or belts meant for steel, they will probably be too hard to shatter and simply go dull. They'll look fine.
A good woodworking belt will actually wear out. Its the nature of the product. Albeit not in 10 seconds to one minute. The quality of the Aluminum Oxide (or whatever the abrasive is) is the most important part of a sanding belt or sand paper.