Dave -

There are separate standards for tape cleaning, but they're difficult to verify so most people who've used tape for truly sensitive info simply burn them. No recycling permitted. Most of us - especially with little tape cartridges - can just lay them on top of the color TV for a few on/off cyles and expect them to be pretty clean; but more sophisticated methods would be better if you're really worried.

A simple degauss affects the whole tape pretty much the same, and it's the differences between nearby spots that matter. These differences can be pulled up after some pretty strenuous application of bulk degaussing. The last time I looked, DoD Tempest required you to actually write bit patterns repeatedly to meet "secure clean" requirements - but that was 20 years ago or so.

John