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I read John Barrett's post on surface preparation for bonding noting his
observation that for dental cement, a fine abrasive making more surface area
is now preferred, providing better bonding.  

This supports what I heard at a composite repair course put on by a company
that repairs composites for airliners.  They also recommend using a fine
grade sand paper (240 to 320 as I recall) to provide maximum surface area
with minimum damage to underlying fiber.  They wipe with solvent (twice_
prior to sanding and vacuum with a clean brush to remove dust.  Bonding
within 4 hours of sanding is recommended because the just-sanded surface
bonds better than one that has been sitting for a while after sanding.  (It
was described as a "high energy" surface.  I leave explanations to the
surface scientists.)  

Grinding away with 40 grit seems intuitively superior, but apparently
science, experience and testing have proven otherwise.

Fred
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