I used to work for a company that made tape erasing equipment and just found a big ole unit in storage that I hadn't seen in years. I'm not real familiar with these and I was looking for some information on the best way to erase used tapes. I found this cool old (1997) site and it even mentions my old company. They moved on but there's still some good information if your at a thrift and pick up an big ole eraser. http://audio-restoration.com/erase.php