Ever demag a cart this way?

Firstly, check with your cartridge manufacturer before using this or any demag. technique. I have a Benz, and they said it was fine, so I use my fluxbuster to great advantage.

As Stringreen says, check with your cartridge manufacturer first. Quite a few (Van del Hul and ZYX for instance) will void your warranty if any active demagging has been done.

Cardas LP demag sweeps. Simple, harmless, cheap, effective.

Will and Test Record sweep that goes from 20-20,000hz deguass the cartridge (i.e Hi-Fi News Sweep), or does Cardas do something unique?

Yes, I've made various posts on the subject of cartridge fluxbusting over the years, to first the Analog Addicts mailing list, later the Phonogram list. Below is a link where a Phonogrammer archived one such post.

http://www.audioasylum.com/audio/vinyl/messages/61192.html

regards and hth