On Sat, 24 Apr 2004 23:14:11 GMT,
wrote:

The reason for the
sandpaper in the patch kit is unknown to many of those who write the
instructions, something that is apparent from the advice to "rough up
the tube" before applying glue.
It is more important than ever to roughen the surface to remove
oxidation and antistick materials like powdered mica, and antioxidants
that migrate to the surface such as hindered amines and cyanuric acid
salts, etc.
In recent years, the solvent in the cement, which used to be benzene,
has been replaced with 1,1,1-trichlorethylene or perchloroethylene to
Save The World From Cancer.
The modern cements are not as agressively solvating to the rubber as
the benzene products were. It was always good practice to scuff the
rubber, but now it is more needed.