If I rub it in sandpaper then it's sharp edges will become round in my opinion.
Cutting the eraser means wasting the eraser.
Can plastic erasers be cleaned by soap or any such thing by which I can protect the sharp edges.
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Sign up to join this communityMy personal choice is to use a blank piece of paper and "erase the dirt" from the eraser onto the paper. If you have shaped your eraser with a knife or sandpaper, this method will be the least destructive.
There are other methods found online. Most curiously, I found a WikiHow article "3 Ways to Clean an Eraser" that describes using a borax solution to clean an eraser. The article is mostly aimed at rubber erasers, but I don't know if the borax solution would work on plastic erasers.
Dishwashing liquid and cold or slightly warm water works.
If that doesn't solve the problem, old fashioned white toothpaste can be used to sand/buff the surface, just rinse well when you're done.
Don't use hot water as it tends to soften the plastic, and let the eraser air dry for at least an hour after washing.