This has been discussed many times in radio forums. Some people apply Ambroid cement, mixed in 50% solution with acetone, to stabilize the asbestos and prevent future flaking, etc.
Here is one discussion:
http://groups.google.com/group/rec.a...e3989b11046a9b
Ambroid is heat-resistant, and apparently good for this application.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html