Renew Old Pencil Erasers

I'm old school, and still draw with pencils.  Something that irks me about pencil erasers is they become oxidized, and hard, don't work no more...  Through some experimentation I found that this oxidized layer is maybe 50 mils thick, and once it is removed the eraser is supple again, and can be use effectively on paper once again.  The process is to uniformly scrum off the oxidization with a moderate grit sand paper, and the eraser is happy again.  I have hundreds of old pencils that are useful again...