I was thinking the other day that because the erase head on a recorder whacks the tape with a high frequency AC current to scamble the ferrites up, good erasing using the erase head might be more effective at speeds the exact opposite to good recording. That is, the slower the tape speed through the erase head then the better the erase as the ferrites get more of the, ah, 'treatment' than they would at the speeds we would normally record at.
I've been putting some of my 7" reels through the Nagra full track erase heads at 3 1/2" ips, sometimes twice, to erase them for later use. There's nothing left after that, if you can stand the wait that is.
I used to have some bulk erasers that I remember working pretty well but like a lot of stuff from that time they somehow got dissappeared.