Sorry I am late to this thread but thought I might add some laboratory
findings about the "white powder" issue.

While the white powder may be stearic acid, it can be other things as well.
Laboratory testing on white powder tape residues have shown that it can be
any (or a combination of) the following:
cyclic trimers from the polyester, sodium or calcium impurities from the
magnetic powder, various fatty acids or stearates from the lubricant and
surfactants left over from the manufacturing process. 

A few notes about treatment:

First, Incubation ("baking") can increase the volume of some of these
residues on the tape surface.  As such, if you bake tapes with "white
powder",  you should consider cleaning them again, after baking,  before
playback.  Second, once a tape starts to exhibit "white powder", it can be a
rather persistent  issue.  A test sample of tapes with "white powder" was
cleaned periodically over a number of years.  After 20 cleanings and 5
years, white powder continued to emerge from the tape onto the tape surface.

Peter Brothers
[log in to unmask]
Audio and video restoration and re-mastering since 1983