How can one determine if sandpaper has expired or become ineffective during use? What are the signs?

1. Severe wear:
When you see that the abrasive particles on the surface of the sandpaper have fallen off a lot or worn really smooth, and you can hardly feel its grinding effect, this is usually a clear sign that the sandpaper is no good. After the sandpaper has been used for a while, it wears out more and more. If the wear is too bad, the sandpaper's grinding ability will drop a lot and it might not be able to grind the material surface well anymore.
2. Clogging happens:
If the abrasive particles of the sandpaper get clogged during use and can't grind effectively anymore, that's also a sign that the sandpaper has gone bad. The clogging might be because the abrasive particles are covered by impurities or scraps on the material surface, or it could be because of the material of the sandpaper itself.
3. Uneven surface:
If the surface of the sandpaper is uneven, has cracks or creases and such, its grinding effect will be affected. These uneven surfaces might cause scratches or unevenness on the material surface after grinding.
4. Decreased adhesive strength:
The abrasive particles of the sandpaper are stuck to the base paper with an adhesive. If the adhesive gets old or doesn't work well, the abrasive particles are likely to fall off. When you notice that a lot of the abrasive particles on the sandpaper start to fall off, it's probably that the adhesive has lost enough stickiness.
5. Poor grinding effect:
If when you use the sandpaper to grind and you find that the grinding effect isn't as good as before, or it takes more time and effort to get the grinding result you want, this might also be a sign that the sandpaper is no longer working well.
6. Changes in physical properties:
If the sandpaper is exposed to a damp, hot or direct sunlight environment for a long time, its physical properties might change, like becoming brittle, soft or warped.
These changes can all cause the sandpaper's grinding effect to go down or stop working. To tell if the sandpaper has expired or failed, you mainly need to look at how much it's worn, if it's clogged, if the surface is flat, the adhesive strength and the grinding effect and so on. Once you find any one or more of these signs on the sandpaper, you should think about getting a new one to make sure the quality and efficiency of the grinding work. At the same time, when using and storing the sandpaper, you should also be careful to avoid bad things like dampness, high temperature and direct sunlight to make it last longer.