I found a larger 'VCR' tape eraser at Radio Shack. It does a very effective job blanking out all the various tapes I have tried it on.
One caution is that it has an internal high temp shut-off if you have it on too long and can take quite a while to reset (cool-down). About 60-90 seconds is as long as you can use it continously but erasing any one tape only takes several seconds.
Lefty