Can sandpaper be repaired or reused in some way after it has worn out?

After sandpaper gets worn out, there are some ways you can give a try to fix or reuse it. Though its performance might not get back to brand-new completely, by taking some measures, you can still extend its lifespan to some degree.
The most straightforward way is to check out the wear situation. If the sandpaper is just a little worn and there are still some abrasive particles on the surface that haven't been used up completely, then you can restore part of its grinding ability by simply cleaning off the dust and debris stuck on the sandpaper. This can usually be done with a brush, a hair dryer or compressed air. Remember to do it gently to avoid damaging the sandpaper further.
If the wear is a bit more serious but the core part of the sandpaper is still solid, you can think about "refreshing" the sandpaper. Here, "refreshing" doesn't really mean restoring it to the way it was at the beginning. It's about making the surface rough again through some means so that it can still be used. One way is to use a finer sandpaper or grinding paste to "self-grind" the worn sandpaper. That means using sandpaper to grind sandpaper to get rid of the already smooth parts on the surface and expose the newer and still abrasive parts beneath. Of course, you need to master certain skills and the right amount of force. Otherwise, you might wear it too much and shorten its lifespan instead.
Besides, some creative workers or DIY lovers might try to use glue or other adhesives to stick a new layer of abrasive (like pieces taken from an old grinding wheel or specially bought sandpaper repair materials) on the back of the worn sandpaper. This method needs some manual skills and the effect might be different depending on the materials and the way you operate. But if you're interested and patient enough to have a go, maybe you can find a "regeneration" method for the sandpaper that suits you.
However, it's worth noting that no matter which method you use, the effect of the repaired sandpaper when you use it is likely not as good as a brand-new one. Especially for those applications that require an extremely high surface finish, using the repaired sandpaper might affect the quality of the final product. So, when deciding whether to repair the sandpaper, you need to consider the repair cost and the possible benefits, and also whether the repaired sandpaper can meet your needs when you use it.
Although it's hard to make the worn sandpaper completely like new again, through some smart ways, we can still try to make its lifespan longer and reduce waste. Of course, if the wear is too serious or the repair cost is too high, then replacing it with a new one in time is also a smart choice.